Move the specific data out of SceneItem into separate vectors. This makes the process of
sorting/moving scene items faster (less memory to move) and makes rectangle rich scenes
consume less memory.
This also paves the way for a "SimpleSceneTexture" vs. a more complex texture.
The README.md contains the warning that used to be in lib.rs.
Add README.md files to all internal crates
... pointing to the official public crate to use instead.
Rename internal crates
fixup: README files
fixup rename