Use new plugin-modality functionality in Mutter

Use new plugin-modality functionality in Mutter

We now have functionality in Mutter to grab the keyboard on behalf
of a plugin. This avoids interactions with the key handling code
in Mutter that could leave the user with an inconsistent state
and no way to get out of it.

src/shell-global.[ch]: Change shell_global_grab_keyboard() and
  shell_global_grab_keyboard() to shell_global_begin_modal()
  shell_global_end_modal() and call mutter_plugin_begin_modal()
  mutter_plugin_end_modal() rather than directly grabbing the
  keyboard.
main.js: Call global.begin_modal/end_modal from Main.startModal()
  and Main.endModal()
altTab.js; Remove call to Main.startModal() - we're letting Mutter
  handle modality for Alt-Tab.
main.js lookingGlass.js overview.js runDialog.js: Rename
  Main.startModal() to Main.beginModal() for consistency with
  naming in mutter and ShellGlobal.

http://bugzilla.gnome.org/show_bug.cgi?id=590686