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.

source/blender/editors/include/ED_keyframing.h

@@ -229,6 +229,10 @@ struct KeyingSet *ANIM_get_keyingset_for_autokeying(struct Scene *scene, const c
 /* Dynamically populate an enum of Keying Sets */
 const struct EnumPropertyItem *ANIM_keying_sets_enum_itemf(struct bContext *C, struct PointerRNA *ptr, struct PropertyRNA *prop, bool *r_free);
 
+/* Use to get the keying set from the int value used by enums. */
+KeyingSet *ANIM_keyingset_get_from_enum_type(struct Scene *scene, int type);
+KeyingSet *ANIM_keyingset_get_from_idname(struct Scene *scene, const char *idname);
+
 /* Check if KeyingSet can be used in the current context */
 bool ANIM_keyingset_context_ok_poll(struct bContext *C, struct KeyingSet *ks);