QtCompositor: 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.

Change-Id: I31475e120ae6efd7bdec4dda453c9ebf20099442
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>