position in GTK, so I can now implement the other half of -geometry
which I'd previously believed to be impossible in GTK. It's still
not perfect, because GTK apparently provides no way for us to get
hold of the X reparent event in order to support negative geometries
in a manner which takes account of the WM borders; but for positive
position it's at least an improvement on the previous version!
[originally from svn r3078]