delegate menu actions to ViewController

[ghazel]

The basic idea is to show the menu from the cell, but delegate shouldShowMenu/canPerformAction/performAction to the ViewController, as iOS would do and has methods for on UICollectionViewDelegate.

The ActionTarget hack I'm not terribly pleased with. UICollectionViewCell has an undocumented _performAction:sender: which would be perfect for this, but I don't know if overriding it is kosher with the App Store.

If that's ok to use, then all the ActionTarget stuff and targetForAction can be removed and replaced with a simple:

- (void)_performAction:(SEL)action sender:(id)sender
{
    [self.delegate performAction:action forCell:self withSender:sender];
}

[ghazel]

Relevant to #458

[jessesquires]

hey @ghazel - this is a bit too hacky, but i've got a better solution coming.