Toolkit specific code in seemingly agnostic modules.

[aaronayres35]

This was motivated by #954 (comment)

Basically, the module pyface/dock/dock_sizer.py, given that there is no wx along its path, appears like it would be toolkit agnostic.
Similarly in pyface/sizers/. However, these modules directly do import wx.

We should audit the codebase for such occurrences. I believe in an idea world we would have these modules have a ___ = toolkit_object("_____") and then some separate module eg pyface/ui/wx/dock/dock_sizer.py containing the wx specific code. (I would think this is true even if there is no qt analogue for these sorts of things, but perhaps

[corranwebster]

The dock and sizer sub-packages are indeed Wx-only, but are never going to have Qt equivalents: dock provides a dock pane management system for Wx, and sizer provides new layout behaviours for Wx sizers. They possibly should live in pyface.wx (particularly sizers), but there is an argument that Wx dock should live in its own package.