Action bars: glow proc and reactive spells from ClassicAPI's overlay events

A button on the action bars pulses a gold border while the spell in its
slot glows: Overpower, Revenge, Execute, Riposte, Counterattack, Mongoose
Bite and Hammer of Wrath while usable, and Shadow Bolt under Shadow
Trance. The slot update resolves the spell, through GetMacroSpell for a
macro, and reads IsSpellOverlayed, so glows lit before a /reload show.
SPELL_ACTIVATION_OVERLAY_GLOW_SHOW/HIDE then update the buttons with that
spell ID; every rank fires its own event.

"Highlight Proc And Reactive Spells" under Action Bars turns it off.
The screen alerts are left out: their art isn't in the 1.12 client.

env/translations_enUS.lua

@@ -433,6 +433,7 @@ pfUI_translation["enUS"] = {
   ["Highlight Out Of Mana Spells"] = nil,
   ["Highlight New Items"] = nil,
   ["Highlight Out Of Range Spells"] = nil,
+  ["Highlight Proc And Reactive Spells"] = nil,
   ["Highlight Settings That Require Reload"] = nil,
   ["Highlight Unusable Items"] = nil,
   ["Hold [Shift] to use item."] = nil,