add modifier key to frame skipping shortcut handler

[timlandgraf]

It would be nice to be able to press Shift + [the shortcut for framewise video seeking] to override the "tracking enabled" checkbox. This should seek the video without tracking.

Would be nice even when mouse clicking the seek buttons.

[jutanke]

This seems to be related to a greater effort: BioroboticsLab/biotracker_core#90