Fix T76566: Fix slow FBX import of long animations.

Note that this patches changes how we insert keyframes, since we cannot
use the `'NEEDED'` option of the slower previous code, we may generate
more keys than needed.

This change gives about 60 times speedup when importing heavy animations
though, so think that trade-of is totally acceptable.

Patch by @Hotox, with some fixes and cleanup by @mont29.

Differential: https://developer.blender.org/D7762