Better default size for top-level backside
Backside widgets now retain which side

Since either frontside or backside or both can be on the backside of a
widget.

Also can drag backside to another window. And its 'hide' button displays
the frontside if it isn't visible.
Backside background colour mirrors widget

Easier to distinguish and looks nicer.
Cross-window drag and drop of backsides

Including the top-level backside.