Event registration filter

[Zeksie]

Would speed things up no end if we could do something like:

this:RegisterEvent("SOME_EVENT", "player")
this:RegisterEvent("SOME_PET_EVENT", "pet")
this:RegisterEvent("SOME_UNIT_EVENT", "party")
this:RegisterEvent("SOME_UNIT_EVENT", "raid")
this:RegisterEvent("SOME_OTHER_EVENT", "target")

Not regexp matches as that would slow things down, just something simple to say 'I want this event, but only when it's this type of unit'.

This would allow us to remove from the LUA level a lot of identical string comparisons.

Could even make it more generic, and call the filter an arg1 filter. So that it could work for anything, although special case would need to be made to get it to work on filter names. Perhaps simply match the length of the filter with the arg1 string, so "raid" would filter in "raid1", "raid40", "raid21pet", but filter out anything that doesn't start with "raid".