This patch fixes problems where it was possible to click away from
the run dialog and focus some other window and still have the 
run dialog up but not getting keystrokes.

A remaining problem is that it is possible to Alt-Tab when the
overlay or the run dialog is up, and then Metacity ungrabs and
focuses an application widget; Some Metacity changes to disable
global key bindings temporarily may be needed, or there may
already be some functionality that can be exposed.

===
shell-global.[ch]: Replace shell_global_focus_stage()
  with shell_global_grab_keyboard()/shell_global_ungrab_keyboard()
main.js: Add startModal()/endModal() functions to go modal and
  undo that.
run_dialog.js overlay.js main.js: Use startModal() for the overlay
  and for the run dialog.