Show avatars before game starts
Add TODOs
Get rid of level tiles; just entities
Simplify first_solid_entity
Remove LevelGeneratorBase
Get rid of level's dirty lists

Not sure if this is the right thing to do, but it simplifies things.
TODO
Track Tourney instead of individual Games

I basically renamed Game -> Tourney.  I'll flesh this out more in the
future.
Make simple game state machine
Shuffle priorities
Re-add dirty lists in level

Partial revert of 0b0e305db17faf8eaec6c9d4322b219779968ac0.  I didn't
like the frame rate taking a hit without the dirty lists.