UI Manager fixes and improvements.
UI Manager fixes and improvements.

The UI Manager wasn't deleting its own record of buttons when they
were removed, which led to some buggy behaviour.

Additionally, buttons couldn't have their x/y positions set by other
code -- they now can. There's a mess of raw ints and strings with "px"
in there, so I had to resort to using parseInt.

wcarss/creek,wcarss/creek