Add 'switch-applications' keybinding

Add an additional "switcher" keybinding for switching between
applications rather than windows (like the existing 'switch-windows'
and 'switch-group' bindings).
The purpose of the new keybinding is to be taken over by gnome-shell's
application-based alt-tab popup, so rather than actually implementing
an application switcher in mutter, let it duplicate the normal window
switcher when run standalone.

https://bugzilla.gnome.org/show_bug.cgi?id=688913