front: Enabled auto-hold feature on stock touchscreen skins.

There is currently no visual feedback on the stock skins and older
custom skins that do not follow the new extensions to the skinning
protocol. I'll add some simple fallback images in a future commit to
provide some basic visual capability.  Probably use a red dot or
something that's compatible with most skins.

Also moved the auto-hold image stuff to VisibleTouchMap, since that's
where it really belongs. Auto-hold images have nothing to do with the
touch map itself, as they are simply visual indicators, like the FPS
indicator and the moving joystick head.