Toolkit specific code in seemingly agnostic modules.

This was motivated by https://github.com/enthought/pyface/pull/954#discussion_r654393738

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