Adds a new scene tool setting `use_grease_pencil_multi_frame_editing`.
The `foreach_*_drawing` functions are moved to the `ed::greasepencil` namespace in the editor, since they are now context sensitive and depend on the toolsetting. They are now named `retrieve_editable_drawings` and `retrieve_visible_drawings` and return
an array of drawings instead of calling a callback function.
Pull Request: https://projects.blender.org/blender/blender/pulls/114283
Falk David
91db8fc5a0