Adaptation of the operator `ACTION_OT_keyframe_insert` to take into account grease pencil channels.
Grease pencil keyframes can now be inserted in blank mode, or in additive mode (duplicate the current frame in the new keyframe).
Two API functions were added :
* `add_duplicate_drawings` which copies the data of a drawing to creates a duplicate, and
* `insert_duplicate_frame` which creates a duplicate frame of an existing one either by copy or instance.
The additive mode option is also added to the UI.
The Layer method `frame_key_at` previously private was set to public.
Pull Request: https://projects.blender.org/blender/blender/pulls/110649
fd75695bed