Basic support for animation frames

[irydacea]

It would be nice to be able to open image files belonging to a single unit animation together, in order to preview the effects of colour transforms on each frame while still grouping them up in a logical and intuitive fashion.

This is somewhat connected to issue #6, which deals with switching to a MDI architecture in general, but separate. The goal would be to be able to open multiple image files either as independent units, or grouped up neatly into animations, with sliders or comboboxes to switch between frames in the same document tab.

(A crazier long term goal would be to replicate Wesnoth's animation engine but it doesn't seem particularly feasible at the moment, especially since it would 100% require us to gain the ability to consume WML.)