Animations: absolute alpha and setToFinalAlpha, combat feedback text fades on an animation group

Combat feedback text fades in, holds and fades out on an animation group
on the font string instead of an OnUpdate helper frame. Autohide and the
XP/rep bars keep the faded-out alpha with SetToFinalAlpha instead of an
OnFinished that sets it. Pulses, the mail blink, the holiday dusk and the
candle flicker give their alphas with SetFromAlpha / SetToAlpha.

modules/xpbar.lua

@@ -381,10 +381,10 @@ end
 
     if not b.fade then
       b.fade = b:CreateAnimationGroup()
+      b.fade:SetToFinalAlpha(true)
       local fade = b.fade:CreateAnimation("Alpha")
       fade:SetChange(-1)
       fade:SetDuration(.3)
-      b.fade:SetScript("OnFinished", function() b:SetAlpha(0) end)
     end
 
     b:RegisterEvent("FACTION_STANDING_CHANGED")