pointer motion signal and default grab handler override

[Giulio Camuffo]

Hi,
this series implements two mechanisms to allow implementing things
like hot corners in a shell plugin.
It adds a signal to weston_pointer which gets firef when the pointer
moves and adds a way to override the default grab. The motion handler
is now in charge of calling weston_pointer_move() to actually move the
pointer, so pointer barriers can be implemented.