Make surfaces visible by default

Also be explicit about the visibility, always send the notification
when a surface is set to QWaylandSurfaceItem

Change-Id: I3507399620bfcc078f7dcd6319f2b55ed1c1f3ad
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Fixed uninitialized member variable
Merge pull request #2 from mikkoharju/expose

Pull in Make surfaces visible by default, fix initialization