Grease Pencil: Expand selection by stroke order across selected frames.

When using multi-frame edit mode in grease pencil, often times a user wants to select the same stroke that may have been generated by the interpolation tool. Selecting the same stroke is not easy when they start to overlap, and there is currently no way to select by stroke order.

I made a very tiny addon that does exactly this by adding a new selection operator that selects all strokes with the same order of already selected strokes that are in selected keyframes. This expands the selection to include possibly interpolated strokes.

To my surprise, I have not seen this suggested anywhere despite its utility and simplicity. I think this feature would be very useful for workflows that heavily use the interpolation tool since it allows for the same stroke on all selected keyframes to be selected without having to pick it out through a jumbled mess. It is also useful for visualizing stroke order and quickly adjusting already interpolated frames without having to delete and interpolate again, or meticulously edit them.

Here is a video demonstrating how the operator would work: