Add transient parent and offset to QWaylandSurface

Compositors now know more about transient surfaces.
With the parent and offset information, compositors can place transient
surfaces relative to their parent.

Change-Id: I73e08aad7bb1d8a7d40b2006208eb48e2a1a8455
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>