Add user labels/predictions to suggestions

[talmo]

In large projects with many labels or predictions, it can be difficult to keep track of all of the labeled frames, especially after multiple rounds of labeling, when you have multiple annotators, and other non-standard workflows.

The suggestions table helps when it's used, but for some cases there may be labels that aren't included in that table.

#736 would help, but until we converge on a good solution for browsing through all labels, a good workaround would be to have the ability to manually add frames to this table for ease of labels management.

This could work by having options under the Labels menu to "Add user-labeled frames to suggestions" and "Add predicted frames to suggestions".

The logic would basically be:

for lf in labels:
    if lf.has_user_instances:  # or lf.has_predicted_instances
        labels.add_suggestion(lf.video, lf.frame_idx)