Added Managers back in. Removed references to sheets in UILayer.

Sheet management to be under sole domain of SheetManager, including
their position and such.

Signed-off-by: Ken Hoover <ken.hoover@mail.utoronto.ca>
Backgrounds now loaded by AssetManager.

Cleaner, can let their loading wait.

Signed-off-by: Ken Hoover <ken.hoover@mail.utoronto.ca>
Added Group and Person classes.

Models & renders individual people/groups.

Signed-off-by: Ken Hoover <ken.hoover@mail.utoronto.ca>