KOTOR: Add GUI buttons loading support.

The loading code streched out the checkbox and made it too large, so I added a
scaling factor that made the image match up the vanilla version's and
made the image extent equal to the height reported in the gui file in
order to make the checkbox round.
KOTOR: Fix coloring of button widgets.
KOTOR: Add button mouse over highlighting.
KOTOR: More button color fixes.