Add interpolation curves.

This was @Victorious3 , and @Victorious3 wanted me to make the issue for it just for me to assign them to it out of some love of organizational overhead.
This will likely either require replacement or modification of the AnimSet format, check the relevant files for details and modify as required.
Not sure how the GUI should look.
Given that you may want to animate different things independently, this may actually require a restructuring of the AnimSet system. Right now it's simply an extension of the PoseSet system, in a way.
Either you could treat the whole transition with the same curves (not good, but UI is a LOT simpler),
or you could consider replacing the timeline & controls with a proper IPO editor (which makes it more difficult to use, because now each bone - potentially each property - has, essentially a different timeline)
https://dl.dropboxusercontent.com/u/21184720/notes.jpg

[Victorious3]

Each bone having its own timeline makes sense, that's how MMD does it as well.

It has four different interpolation curves, one for each axis and one for rotation. (64 bytes encoded in here, so 4 floats each: http://mikumikudance.wikia.com/wiki/VMD_file_format)

It's just a quadratic bezier curve:

It also makes things more complicated, but we'll see from experience, I suppose.