Restructured the event loop; screens can now interact

Moved most of the loop into the screen parent object. Also the clock now
ticks at the end of the loop.

The menu screen now calls the game screen, which exits back to the menu
when its loop terminates.

Also moved level management completely into the game object, so that on
death it can reset the level rather than exit the program.
Rename main class to Window, create new start script
Add level selection to the menu; a bit more restructuring

Menu now lets us pick a level to start playing on.

Put frame update/clock tick back at the beginning of the screen
loop and removed it from init, because it was redundant.

Also did a bit of cleanup on the method calls for when the
window is resized.