Refactor Layer -> Element

[NGimbal]

"Layer" terminology is unfortunate as Layer should actually refer to a higher layer of abstraction group of objects on a layer of the drawing. Element is the currently winning idea for what to call this. Other ideas are:
Leaf
Object
Item
DrawObject
Plane
DrawQuad
Glyph

I kind of like the Leaf idea but it is sort of obtuse. Glyph is nice but also a little too obtuse... maybe the most accurate. Although might become overloaded with font rendering in the future? Likely each character would be rendered on its own bounding quad? DrawQuad - what if I implement clipping at some point? Maybe it should be leaf or glyph...