ClassIcons tweaks/fixes

* When the PLAYER_TALENT_UPDATE/SPELLS_CHANGED events were unregistered the element effectively got disabled completely if the player changed specs back and forth.
* Use SPELLS_CHANGED for all classes to only show the element when the player can use the resource for something, or if the resource is available to them (like Priest obtaining Shadow Orbs at level 21)
* DARK_FORCE for Monks has not been used since the MoP beta (afaik), removed it and cleaned the code that dealt with multiple power types.
* Added a spell check for Paladins so the element doesn't show until they can use holy power (Word of Glory is their first spell, learned at level 9)
* Replaced the VehicleToggle function to prevent the element getting falsely updated during load and would show the element regardless if the player had the correct spec/spell.
* Other minor cleanups due to all the changes required, re-used some variables where it fitted.