units: Adding two generic boss events.
power: optimize a bit
altpowerbar: fixed visibility.

Always show on player.
Show on group units if showOnRaid is true.
Show on any unit unless hideFromOthers is true.
stagger: add monk's stagger bar
stagger: minor optimization.
stagger: update on UNIT_AURA instead of using an OnUpdate script.
stagger: stance-based visibility.
stagger: added a hook to override the internal visibility function.
stagger: small optimization.