Power menu attribute replaced_by_power

This attribute is designed to be used when there are multiple versions of the same power e.g. attack1, attack2, attack3 etc. In this scenario, use replaced_by_power to mark attack2 as a replacement for attack1 etc. The effects that this has:

1. for active powers, the replaced power can no longer be used
2. for passive powers, the replaced power no longer grants the passive bonus
3. for active powers, the replaced power cannot be dragged to the action bar
4. if an active power is unlocked it automatically moves to the action bar over the replaced power, if the replaced power was in the action bar.
5. a tool tip displays "Newer power available" on the replaced power

The main reason for wanting this attribute is to avoid unintended advantages gained from having multiple level versions of the same power available e.g. a long cooldown for "uber attack level 2" could be bypassed by using "uber attack level 1" straight afterwards. For spawn powers, the spawn limit could be bypassed by using multiple versions of the power. This attribute prevents these exploits.