libweston-desktop: Prevent spurious focus change signals

This should prevent extra focus signal emission the similarly to
how default_grab_pointer_focus() does, though we don't have the
surface jumping logic here.

This stops xdg pings from being sent every output repaint during
a grab when the pointer isn't in any windows belonging to the
grab parent.

An example would be running weston-terminal, bringing up the
right click pop-up, and moving the mouse onto the desktop while
another client causes repaints.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>