OverlayVisRep, like ArenaVisRep.

Moved all the OverlayTextLineVisRep creation/updating/drawing/teardown into an aggregate OverlayVisRep, much like ArenaVisRep and the Background and Ring VisReps.

• New OverlayVisRep class.
• OverlayTextLineVisRep storage and handling moved from SceneForCG and SceneForGLK to OverlayVisRep.  SceneFor…s' drawOverlayTextLinesForView methods removed as a result.
•Scene is now creating, freeing, and owning the OverlayVis just like for ArenaVisRep.
• OverlayVisRep has a bounds property set by SceneForCG/GLK's drawOverlayLayerWithin: to a rect with 0x0 origin and the width and height of the view's bounds (and won't draw if it hasn't been set yet).
	» I couldn't think of a cleaner way to do this right now.  And I'm antsy.
• In SceneForGLK, the same erase-and-reinit hack for now that's necessary for ArenaVisRip because I'm a bad, bad horrible person.  Also because CoreGraphics and OpenGL are quite a bit different, and I haven't fully modeled everything by intent yet— so back to the first reason.
• Added OverlayVisRep to project file.

Also:
• Replaced the hard import for ArenaVisRep in the Scene header with a forward declaration and added imports for ArenaVisRep (as well as OverlayVisRep) in necessary .m files.
I think that just about wraps up this refactor.  In spades, Duck, in spades.