Bug fixes, minor changes

So Ludum Dare #31 is happening, and there are bugs! At least the ones I
found until now are getting fixed with this commit though.
Also, a few minor changes were done regarding some methods.
* Screen class now has a default rendering behavior (it renders like a
normal GUI container). It also has a shape, so a background can be set.
* TiledLevel provides 3 new methods, for checking whether tile
coordinates are in bounds as well as getting the raw tile code at a
location.
* The load() method that also takes a properties file path was moved
from TiledLevel to Level. Also, it now already loads the properties,
instead of only constructing the object.
* Chaser instances no longer move like an entity because that just
didn't work. They do not have a velocity any more, rather they keep a
target to which they move.
* Mouse coordinate retrieval was fixed when in fullscreen mode. Maybe
this is a temporary solution and will change again, but at least it
works.
* Components take their bounds from the shape, not the opposite.
* A Display also has bounds now.