Skin change keys

Some challenges:

- We need to define (or make configurable) how the attachment visibility changes when the skin changes. Current runtime `setSkin` behavior only sets attachments from the new skin if the corresponding attachments in the old skin were attached.
- This feature will be less powerful, but still useful, if it can only show/hide a single skin. It is more complicated to allow it to combine skins like the `Skins` view, as each key needs a list of skins in the order they should be combined. Ideally at runtime there would not be allocation when setting a combined skin for each key.
- How do AnimationState tracks work with skin change keys? Does the last applied track overwrite any skins set previously?
- How can we handle if the user wants to set some skins manually, but use skin change keys to set or unset some other skins?

http://esotericsoftware.com/forum/viewtopic.php?f=11&t=3410&p=16674