Reorganize Tools

Related to #438
Pulling the contents of #739 into an issue:

This is the current starte of Pan and Zoom tools accross Chaco and Enable (red = in enable):

```
digraph OldToolInheritanceTree {
BetterZoom -> DragZoom;
BetterZoom -> BetterSelectingZoom;
ZoomTool [
label=Currently just an alias for BetterSelectingZoom>
];
BetterSelectingZoom -> ZoomTool [dir=none];
RectZoom [
label=Just sets 2 traits on ZoomTool>
];
ZoomTool -> RectZoom;
TrackingZoom [
label=Defines one additional method on ZoomTool>
];
ZoomTool -> TrackingZoom;
DragTool [fillcolor=red, style=filled];
DragTool -> DragZoom;
BaseZoomTool [fillcolor=red, style=filled];
ViewportZoomTool [fillcolor=red, style=filled];
ViewportPanTool [fillcolor=red, style=filled];
DragTool -> ViewportPanTool;
BaseZoomTool -> ViewportZoomTool;
PanTool;
TrackingPanTool [
label=Defines one additional method on PanTool>
];
PanTool -> TrackingPanTool;
PanTool2 [
label=Not in chaco.tools.api, very rarely used.
However, seems to have been intended as improvement over PanTool.>
];
DragTool -> PanTool2;
}
```


Until recently, there had also been a ``BaseZoomTool`` and ``SimpleZoom`` living in chaco. These now exist in enable (with some trimming / modifications) and are the ``BaseZoomTool`` and ``ViewportZoomTool`` above. Additionally, we recently removed the duplicate of the enable ``DragTool`` which was living in chaco.

``RectZoom`` and ``TrackingZoom`` are intended as convenience classes but end up clouding the api. Instead we should simply have a ``ZoomTool`` class (which is actually ``BetterSelectingZoom``, but that code can be copied over and ``BetterSelectingZoom`` removed). This is the "default" / go-to zoom tool most users will use. If they want the current ``RectZoom`` or ``TrackingZoom`` functionality, there should simply be an easy way to configure a ``ZoomTool`` to do so. Either by setting a trait like ``rect=True`` or ``tracking=True``, or perhaps with some class method on ``ZoomTool``. This way it will be more obvious what tool you want if you want zoom functionality (you want the ``ZoomTool``!) and it can be confiugred to your needs. Similar logic applies to ``TrackingPanTool`` and either ``PanTool``. Although currently, ``TrackingPanTool`` subclasses ``PanTool1``.

The current ``BetterZoom`` class can be renamed as ``BaseZoomTool``. The way things currently are (with ``ZoomTool`` an alias for ``BetterSelectingZoom``), the "default" zoom tool has selecting functionality. There are situations like ``DragZoom`` where you don't want this. AFAICT, users are not expected to use ``BetterZoom`` directly. As such, it makes sense to make it an explicit base class.

In enable, I do not really see why ``BaseZoomTool`` needs to be its own class. It is only used by ``ViewportZoomTool``, and is not exposed in any api module. Further, from what I can tell, enable zoom functionality is only possible via a ``Canvas`` with a ``Viewport``. So only having a ``ViewportZoomTool`` seems reasonable. However, if the class is anticipated to be used as a base class for other zoom tool variants, ``BaseZoomTool`` can easily stay.

Additionally, either ``pan_tool.PanTool`` or ``pan_tool2.Pantool`` should be removed. (I still need to investigate the feature disparity / advantages of one over the other, if any exist)

After some initial investigation it is clear much of the code is copy pasted. However, some obvious differences include:

* PanTool1 subclasses ``BaseTool`` whereas PanTool2 subclasses ``DragTool``
* PanTool1 has ``pan_{right/left/up/down}_key`` traits, PanTool2 does not
* PanTool1 also allows "middle" for ``drag_button``, PanTool2 (inheriting ``drag_button`` from enable ``DragTool``) only allows "left" or "right"
* PanTool1 sets event state to "panning" and defines "panning" specific methods whereas PanTool2 overrides methods on DragTool (e.g. ``dragging``, ``drag_cancel``, ``drag_end``)

Potentially relevant issues include enthought/chaco#519, and enthought/enable#90.

The following is a proposal for a new class heirarchy:

```
digraph NewToolInheritanceTree {
BaseZoomTool [
label=Formally BetterZoom>
];
ZoomTool [
label=Formally BetttterSelectingZoom / ZoomTool>
]
BaseZoomTool -> ZoomTool;
BaseZoomTool -> DragZoom;
DragTool [fillcolor=red, style=filled];
DragTool -> DragZoom;
ViewportZoomTool [fillcolor=red, style=filled];
ViewportPanTool [fillcolor=red, style=filled];
DragTool -> ViewportPanTool;
PanTool [style="dotted"];
PanTool2 [
label=Not in chaco.tools.api, very rarely used.
However, seems to have been intended as improvement over PanTool.>,
style="dotted"
];
DragTool -> PanTool2;
}
```


Migration Steps:

1. Rename ``BetterZoom`` as ``BaseZoomTool``
2. Copy ``BetterSelectingZoom`` into ``ZoomTool`` and delete old ``BetterSelectingZoom``, or delete old ``ZoomTool`` and rename ``BetterSelectingZoom`` as ``ZoomTool``
3. Decide on means for replacing ``RectZoom`` and ``TrackingZoom`` and with functionality on ``ZoomTool``
- ``RectZoom`` currently just subclasses ``ZoomTool`` (aka
``BetterSelectingZoom``) and sets ``tool_mode = "box"`` / ``always_on = True``
- ``TrackingZoom`` currently just subclasses ``ZoomTool`` (aka
``BetterSelectingZoom``) and defines a ``normal_mouse_wheel`` method.
4. Do the same for ``TrackingPanTool`` (which just subclasses ``PanTool`` and overrides ``_end_pan``).
5. Chose one of ``pan_tool.PanTool`` and ``pan_tool2.PanTool`` to be the go-to PanTool moving forawd. Update as needed / Delete the other.
6. Decide fate of ``BaseZoomTool`` in enable.