Combo update

- Up to 3 (by default; configurable) previous hits display their damage
in the Combo display
- Combo HUD display shows immediately on first hit when locked on,
rather than on the second hit
- Combos don't end until hit timer runs out, even if not currently
locked on
(only hits that occur while locked on count towards the combo)
- Significantly reduced the amount of time allowed between successive
strikes to 20 + 2 per level (from 25 + 5 per level)
- Indirect damage (such as Leaping Blow) no longer benefits from combo
bonus damage, nor does it display individually, but the amount will
still contribute to the combo's total damage
- Initial work on targeting icon