This is a PR that is built on top of #122094 (thanks to @SietseB
for the initial work).
Adds the following properties and functions:
* `layer.frames`,
* `layer.frames.new(frame_number)`
* `layer.frames.remove(frame_number)`
* `layer.frames.copy(from, to)`
* `frame = layer.get_frame_at(frame_number)`
* `frame.drawing`
* `frame.frame_number`
* `frame.select`: the selection state of the keyframe in the dope sheet
* `frame.keyframe_type`
* `drawing = frame.drawing`
* `drawing.type` (`DRAWING`/`REFERENCE`)
* `drawing.user_count`: The number of keyframes that use this drawing
* `drawing.attributes`: attribute read/write access to the drawing data
To be able to access attributes on drawings, a new
`AttributeOwnerType::GreasePencilDrawing` is added.
The API in `BKE_attributes.h` is updated to handle this type.
In `rna_attributes.cc`, there is a new
`rna_def_attribute_group_grease_pencil_drawing` that defines the
attribute group. For this to work, it also defines its own rna
callback functions.
Pull Request: https://projects.blender.org/blender/blender/pulls/124787
6f050fecfb