Entangled dependency of TraitsUI and Pyface

Currently Pyface has non-optional dependencies on TraitsUI. After some work, these have been reduced down to:

* `pyface.dock` - this is Wx-only, but is core to the TraitsUI View implementation for that toolkit, and is also used substantially in the Workbench implementation.
* `pyface.workbench` - this is cross-backend, and is used by Mayavi and Envisage, but not by TraitsUI.

But these dependencies are fairly substantial (eg. things like menu handling, building complex dialogs, etc.), old, not actively developed, and seem like they would be non-trivial to resolve within the Pyface codebase alone.

Proposal:

* we move `pyface.dock` to `traitsui.wx.dock` (or _maybe_ somewhere else in TraitsUI, if we need to avoid circular imports). This makes the situation with `pyface.workbench` no worse, but there is no known use of `pyface.dock` outside of the Wx backend of Workbench and TraitsUI (and if there is, they can just adjust imports as needed, since they will already depend on TraitsUI). This change will require adjusting imports, but should otherwise be transparent.
* we move `pyface.workbench` to `apptools.workbench`. This seems reasonable, since Apptools already has substantial dependencies on TraitsUI, and Workbench is precisely the sort of thing that Apptools is supposed to contain - libraries for supporting substantial GUI applications.