Add WindowPixmapItem opaque property

And make the item non-opaque by default.
Add a close client method to compositor

Ideally we'd like to request that just the window close and let the
client figure it out, but currently there is no way to do that.
Add WindowPixmapItem radius property
Bump version
Update changes file
Merge pull request #42 from aaronkennedy/master

Allow translucent WindowPixmapItems