Replace costume when type changed during edit

[coproc]

As discussed with bromagosa at SnapCon I propose these changes to fix issue #2492 ("Editing costume type (SVG <-> bitmap) not working properly").

The following will improve:

• new costumes will always have a unique name of the form 'Untitled' or 'Untitled(1)' ... (currently new SVG costumes will have default names of the form 'costume1', 'costume2', etc. not even guaranteeing a unique name)
• when editing a costume and changing its type while editing, the original costume will be changed as expected in all cases (currently when editing a SVG costume and changing its type to bitmap, the original costume will remain unchanged, although the sprite will wear the changed costume; and when editing a bitmap costume and changing its type to SVG the edited costume will be added as new costume)
• the code of VectorPaintEditorMorph does not depend on the Costume class any more
• the Costume class does not depend on PaintEditorMorph or VectorPaintEditorMorph any more

[bromagosa]

Just to recap, this fixes #2492, among other issues :)

(thanks @coproc!)