Got rid of a few layers of unnecessary widgets
* ApplicationWindow._create_body and TaskWindowBackend.create_contents aren't needed for wx because the AUIManager handles the layout
Added convenience import pyface.wx.aui to load the AUI toolkit
* prefer the pure python version wx.lib.agw.aui (no seg faults!)
* can select toolkit version by setting environ var ETS_WX_AUI to "wx"
Initial support for a single dock pane
* not sized correctly and not hidden by default, but it's displayed at least