Compositor: provide a way to update the clipboard on a client

Clipboard synchronization happens when a client is keyboard-focused, but
some clients may require clipboard access even when not in focus. Add
an updateSelection() slot to QWaylandSurface so that the compositor
implementor can synchronize the clipboard manually or through a
signal-slot connection.
Merge pull request #33 from xknight/master

Compositor: provide a way to update the clipboard on a client