Window moving and resizing depends on the `meta_wayland_seat_get_grab_info`
function succeeding. At the moment, tablet tools do not generate implicit
grabs like the pointer and touch. This commit adds the necessary elements
to track implicit grabs and retrieve their information.
https://bugzilla.gnome.org/show_bug.cgi?id=777333
Each tool has its own MetaCursorRenderer instance, which is created/destroyed
upon proximity, and possibly updated through focus and set_cursor calls in
between.
