Reworked widget handling logic

Widgets now respond to WM_COMMAND messages by implementing
_on_wm_command.
Fixed control identifier extraction
Merge branch 'master' of github.com:ctoth/toga-win32
Added the WS_TABSTOP style to all controls so they can be tabbed to by default
Merge branch 'master' of ssh://github.com/camlorn/toga-win32
Minor code cleanups to comply with pr requests.
Merge branch 'master' into text_widgets