This modifies the 3D viewport keyframing operators to work with layered
actions. The functionality is relative basic, and still leaves some things out.
Of particular note:
- Keyframing with keying sets does not yet work.
- User preferences such as the `XYZ to RGB` flag and the keyframe interpolation
type are not yet used/respected.
- Something not caused by this PR but revealed by it: when the last keyframe of
an fcurve is deleted in a layered action, the fcurve still sticks around. This
is different from legacy actions, which delete fcurves when their last key is
deleted. Since the "Only Insert Available" feature is based on the existence
of fcurves, that makes the feature appear broken in some circumstances right
now with layered actions.
Pull Request: https://projects.blender.org/blender/blender/pulls/121661
a36599b323