WIP: Remove shell_global_sync_pointer

Since we have a proper Clutter API for manually updating cursor focus and triggering repicks now, use it and make sure the actors that are currently focused and not part of our grab stack receive CLUTTER_LEAVE or CLUTTER_ENTER events after the grab began or ended.

With the changes in place in Clutter and Mutter to automatically repick and send crossing events on modals, we can finally remove this issue which caused us lots of pain.

Depends on mutter!429 (closed), mutter!554 (closed), mutter!555 (closed)