Expose the mouse focus functions from Wayland::Compositor

Expose the mouse focus functions from Wayland::Compositor

For Compositors to handle mouse focus handling, ie enter/leave. This
calls down into wayland-server which keeps track of which surface holds
the focus. Trivial implementation in compositor would be to call
this function for all mouseMove events. I believe overhead should not be
significant.

Change-Id: I8fdd5115cc512562c1a8d8b90c28f8ef2d5b53c9
Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>