Better communicate the difference between slot visibility and keyed slot attachment.

http://esotericsoftware.com/forum/Slot-visibility-between-animation-7835

Whether or not this is OP's problem, it's still a point of confusion.
The tree makes it look like slot visibility is keyable because the key icon is right next to the visibility dot.

Proposal:
1) For consistency, make none of the visibility dots in the tree signify anything keyable.
2) Maybe move the key to the right of the slot name. Or show the key button next to the active attachment.
3) When a slot is expanded, its list of attachments should show radio buttons instead of a visibility dot. Probably better not to have it on the visibility dot column. Have one extra item with a gray color: "empty"

Considerations:
- The design and implementation of the new slot attachment image picker. Maybe that can be the sole place where you key attachments.
- I still think it's too much cognitive load to have bones, slots and attachments all under the skeleton hierarchy. I'm partial to the idea of having the attachment selection be a popup menu only when you click something, or another panel/window.
**Low-Hanging Fruit:** At the very least, I think a better default behavior for the tree is to only show the attachment list for the slot you currently have selected, and auto-collapse other slots.
And maybe the show-all-attachments-for-all-slots mode can be a special mode. I think this is especially true for Animate Mode, but it would still be useful in Setup Mode.