WowAce

Hendrik Leppkes:
    - Add a GetBindingAction function to export the binding action used by this button
    - Fire a callback when the keybindings of a button change.
    - Add a function to move buttons to a new header.
    - Split Usable/Cooldown update between "action" based buttons and other buttons.
    This avoids a double update of all buttons, improving performance.
    - Remove varargs from update function, its not used anywhere
    - UpdateFlyout is called implicitly by Update, no need for an explicit call
    - Update copyright year to 2013
    - Fix "empty" typo in secure state change handler
    Thanks to Adirelle for noticing