Splash

Added a simple loading/splash screen which instructs the user to click in order to start playing, and informs them of the controls.

We'll probably want to make this fancier later, with a loading bar and such, and maybe make it wait for a few chunks to load before claiming the world is loaded. However, it seems to work for now.

This pull request depends on the health pull request. They could be separated, but since they both changed a bunch of the same files, I figured this way would result in a cleaner merge.