FIX: make volup/voldown pure "digital" actions

VolUp / VolDown are broken as analog actions, both for gesture and joystick, because GetAmount() contains pears or apples depending on the event origin. 

Rather than trying to get analog working with dirty hacks, with limited use case, let's make them pure digital actions.

Tested with swipe gestures on droid (although that leads to an unrelated bug that swipe and inertial scrolling are conflicting each others) and XBOX360 joystick on Win.

/cc @jmarshallnz @Montellese @Memphiz