GPencil Shape Keys; it's not a duplicate I swear

This is the fifth proposal for Grease Pencil shape keys, but the first one was the only one to explain it in detail, and my proposal is different from theirs so I don't think that this is a duplicate.

Shape keys should be created and controlled at the GP object/data level, but defined per frame

You should be able to add, remove, and control all shape keys in the object from the data tab in a shape keys panel, like you would with a mesh, but the data for the basis frame and shape keys would be stored per frame. Some proposals have suggested that the shape keys should be tied to each layer, but I think that would be a mistake for multiple reasons. Shape keys should have its own panel in the data tab.

Frames shouldn't have shape keys defined for them until they are changed

Frames will only have shape key data when their strokes are changed in edit or sculpt mode while the shape key is selected in the data>shape keys panel. When that happens, the frame will have that shape key defined, and any more changes will change that shape key instead of the basis. Ideally, the shape key panel would have some kind of indicator of which, if any, shape key(s) the active frame has defined. I'm thinking replacing the shape key icons:

The user should be able to remove the active frame's shape key definition for the selected shape key with an option in the specials menu. There should also be other utility functions to control which frames have shape keys defined as well, such as to select the frames that have the selected shape key defined, or to remove the definitions from all selected frames.

Controlling the shapes

When in object mode, the shape keys can be controlled just like mesh shape keys. When the GP object is rendered, it checks each frame that it renders for shape keys and applies them when necessary and draws the result.

Interpolating

When using interpolate or interpolate sequence between two frames that have shape keys defined, each shape key for the resulting interpolated frame should be interpolated between the shape key definitions of the two outside frames. If only one has a shape key, the interpolated frame should get a shape key that is interpolated between that frame's shape key and the other frame's basis shape.

Shape keys are necessary for quality 2D rigs for a lot of art styles. Right now, I'm using my addon GP2DMorphs, but it's kinda limited and having actual shape keys would be much better.

I believe this proposal fixes all issues that I saw raised in other proposals, and the frame approach is more dynamic and versatile than the layer approach that was proposed before. Let me know if you still see any issues.