Add ability to move viewer tabs

[Kneelawk]

Currently viewer tabs are fixed in place once opened. It would be useful to be able to move them around and reorganize them.

[Kneelawk]

I have this mostly implemented. I just need to:

• [x] Move the tab list logic to its own widget.
• [x] Add tab movement animations. (maybe)

[Kneelawk]

I also decided to begin the move to instance ids with this.

[Kneelawk]

Currently, tab movement animations cause new tabs to shoot from the left side when created. I need to figure out how I want this to actually behave.

[Kneelawk]

Ok, fixed that.