XP gain added to PLAYER_XP_UPDATE

[Selite]

It always struck me as strange that the event returns the unit that fires the event, but not the XP gained.
Not that it's too hard to calculate the gained XP, but I'm all for making my code as small as possible, and I could think of a few applications where it would make life a million times easier.
Ergo; it'd be nice if the arg2 of the event was equal to the XP gain in question.

-Sel.