main: add timestamp parameter to push/popModal

main: add timestamp parameter to push/popModal

Right now popModal() passes global.get_current_time() for
its begin_modal() call.  global.get_current_time() is the
timestamp of the last gdk or clutter event processed by the
shell's mutter process.  These values could potentially be
be too stale to use if pushModal() were to get called in
response to an event by another process.

This commit changes pushModal() to have an optional timestamp
argument, which can be used to associate the call with the
event that initiated it.

https://bugzilla.gnome.org/show_bug.cgi?id=637187