Add ability to set a pile as a region-like default for a scene's background

[JPMeehan]

A consistent piece of feedback is "When I drop a card onto the canvas, I expect it to move somewhere". Probably the best way to do this would be to have a flag for scenes that can be configured for the default drop location, either in the normal scene config menu or as a context menu option.

Technical consideration: Will need to make sure there's no competing region