front: Generalized auto-hold determination, migrated code to subclass.

Whether a button is auto-holdable is now determined from a method,
rather than testing an array for null (separate interface from implem-
entation). This will simplify wiring up a preference for selecting auto
holdable buttons.

Also, moved the auto-hold image overlays and loading from TouchMap to
VisibleTouchMap. Since the images now are merely indicators, they
belong there along with the FPS indicator and the joystick animation
code.