[PATCH wayland] Add a relative_grab request to the wl_shell_surface interface

[Philipp Brüschweiler]

The consensus seems to be that we need relative/raw motion and that it
should be orthogonal to clipping/grabbing. I have now tried my hand at
implementing relative motion by introducing a wl_relative_pointer (no
raw motion yet, but it should not be hard to extend the implementation
analogously). There is no grabbing/clipping support in those patches.

Please comment! I'm especially not sure about how much of the
interface of the "original" pointer should be duplicated (or
triplicated?) to the other pointers.

Cheers,
Philipp