Consolidate backstage styling into one tiddler

[jayfresh]

The styling for the BackStage? area in TiddlyWiki is in StyleSheetLayout?, a shadow tiddler. In TiddlySpace?, there are extra backstage styles in StyleSheetBackstage?, which is not a shadow tiddler.

It would be helpful for me, in the process of creating a theme, if the styles were consolidated in one place; so I think, either:

1. make StyleSheetBackstage? a shadow tiddler in TiddlyWiki and have the TiddlySpace? wikis override it to add their custom styles; 2. put all the TiddlySpace? backstage styles in StyleSheetTiddlySpace?, so the TiddlySpace?-specific styles are in one place