Blender Git Loki

Keyframing: add operators that use keying-set ID's

Unfortunately we can't use insert/delete_keyframe operators in keymaps
because the enums aren't known at the time of keymap registration
and the keying sets are dynamic and use a poll function.

Add a version of insert/delete operators that takes a string
instead of an enum. Needed for D4626.

Also extract int to keying-set into a utility function.