Allow any number of Key or Event input mappings
Give the test scene walls
Set up cube to represent the player, with proper input handling
Add key mapping for the Spacebar
Update top-down to allow swapping input controls on Space

This pointed out a flaw in EntityDB: when a component is added to an
Entity, all behaviors get updated and the entity gets added to the
appropriate EntitySets multiple times, even on behaviors that never saw
any changes (like Transform).