Clean up the state of the Pan and Zoom Tools

**Problem Description**

We currently have a plethora of different Pan and Zoom tools (PanTool, PanTool2, ZoomTool, BetterZoomTool, TrackingPanTool, DragZoom, etc.)

We should audit, find out which ones are working as expected, and make them the default (this might be a breaking change, depending on exactly what is needed).

See also #6.

**Expected behavior:**

Developers should have a clear choice for Pan and Zoom tools to use, possibly with one or two clear alternatives that have distinctly different (but common) behaviour. Tools should be based on Enable tools where practical.