Abilities fix

Using certain UseableAbilities on other characters now properly pulls
them into combat. Whether or not this happens is decided by
UseableAbility::_startsCombat and is calculated after the initial
casting animation of the ability finishes.
Audio fix

Fixed an issue which caused AudioHandler::clearData() to crash when
multiple instances of the same sound are playing.
Ability Tooltips

Added tooltips to the ability icons displayed when a single player
character is selected; these give a high-level description of the
ability, telling the player what it does and how to use it.