Changed the way ApplicationShells are used: now wxMotif

  Changed the way ApplicationShells are used: now wxMotif
creates one ApplicationShell per display, and makes top
level windows popup childs of the ApplicationShell.
  Removed a couple of unused variables from wxApp.
  Replaced some calls to wxGetDisplay with XtDisplay(widget)
or event.xany.display, and some others with wxGlobalDisplay
(the latter changes are just eyecandy).
  Used wxFlushEvents where appropriate.
  Fixed (hopefully) wxFindAcceleratorText and wxFindAccelerator;
for now the new version is still disabled, awaiting further testing.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@20474 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775