Warlock power bar: add an outer glow to full burning embers
WarlockPowerBar: Finaly reworked the element

- Supports prereq spells per spec and green fire
- Hides the element when in vehicle and when disabled
- Colors are in the oUF's color table so the layout can change them
- Simplified update as Glyph of Burning Embers and Glyph of Soul Shards were removed in 5.2
- Changed handling of soul shards (set value instead of alpha)
- power and maxPower are now set to 0 instead of nil when running update for a powerType not corresponding to the current spec
- visibility will now trigger an update, but that might change