Improve moving rigging across skeletons

- Allow copy/paste of keys to items in another skeleton.
- ~Allow modifier so dragging across skeletons can be a copy or a move (`ctrl` for copy, else move). Using duplicate is suboptimal because it changes the names.~ 4.3.68-beta
- Allow preserving keys when dragging across skeletons. How will this work? We could show a dialog with options:
1. Transfer all keys: for each animation with keys, create animations in the target skeleton if one doesn't exist with the same name.
2. Transfer keys for the current animation: only create keys for the active animation, to the active animation in the target skeleton.
3. Don't transfer keys.
- Can constraints be transferred?

User request:
http://esotericsoftware.com/forum/Feature-Request-Copy-keys-between-skeletons-15140

#524 is related.