Experimented with collision shake intensity
Added PhysicsCollision event; Screenshake uses it

Only collisions between physics-enabled entities should cause
screenshake.
Reverted to brute force collision detection

The former solution of keeping a record of entities to check and
removing checked ones from the list caused problems when an entity was
destroyed but was kept in the check list.
Modified test scenario

Screenshake needs tweaking, badly.
Tweaked collision shake to make it less annoying

Now that the screenshake framework is done, tweaking for actual gameplay
can be done later.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	states/game.lua
	systems/graphics/Screenshake.lua