Moved loop back out into window class (again)

Decided this makes more sense, since events like window resize and
sys.exit need to happen within this loop, and that belongs in the main
class.

Also added an escape key event to quit the game to the menu.
Window calls the resize hook on resize, instead of on every frame

It does this by looping through a screen stack. Also, the hook gets
called when initializing the screen.

Now all that logic can go right into the subclasses, and almost nothing
is needed in the superclass.

I know I keep moving this stuff around, but it's pretty trial and error.
Started changes to menu text rendering