IsUsableAction

Returns whether an action is usable.

Signature:

Arguments:

• slot - An action bar slot (number, actionID)

Returns:

• isUsable - 1 if the action is usable; otherwise nil (1nil)
  
• notEnoughMana - 1 if the player lacks the resources (e.g. mana, energy, runes) to use the action; otherwise nil (1nil)