wayland: Set/unset wayland focus on mutter grab/ungrab operations

This ensures that we send the proper leave and enter events to wayland
clients.

Particularly, this solves a bug in SSD xwayland windows where clicking
and dragging on the title bar to move the window only works on the odd
turn (unless the pointer moves away from the title bar between
tries). This happens because xwayland gets a button press but doesn't
see the release so when it gets the next button press it discards it
because its pointer button tracking logic says that the button is
already pressed. Sending the proper wayland pointer leave event fixes
it since wayland clients must forget about button state at that point.

https://bugzilla.gnome.org/show_bug.cgi?id=726123
wayland-seat: Don't send pointer enter/leave events during a GRAB_OP

meta_wayland_seat_repick() can be called in various cases while mutter
has a GRAB_OP ongoing which means we could be sending wrong pointer
enter/leave events.

https://bugzilla.gnome.org/show_bug.cgi?id=726123
wayland: Exempt CLICKING grab ops when syncing wayland input focus

If we have a CLICKING grab op we still need to send events to xwayland
so that we get them back for gtk+ to process thus we can't steer
wayland input focus away from it.

https://bugzilla.gnome.org/show_bug.cgi?id=726123