runAction:withKey: like in Sprite Kit, stops existing action with key

[LearnCocos2D]

This is a multifold request to improve working with actions:

• add string keys to actions (in addition to tags?)
• an improved runAction: that allows to specify key (and/or tag), like runAction:withKey: and runAction:withTag: because it's cumbersome having to assign tag/key to the action instance
• behavior that stops any existing action with the same key (optionally for tags as well) ... for instance: runAction:withTag:stopExisting: and runAction:withKey: where the key variant always stops the existing one

[slembcke]

I'll bump this to the 4.x milestone. Hopefully will get more visibility there.