[Feature] Add ability to regenerate thumbnails (or other generated files) for a specific video

[victorhooi]

Is your feature request related to a problem? Please describe.

Sometimes, the thumbnails for a video might be corrupted (e.g. video was being downloaded or transferred at the time).

Describe the solution you'd like

It would be useful to be able to re-generate the thumbnails (and other generated content) for a selected video on demand.

Possibly via a right-click menu, or maybe a button when you open the video.

Describe alternatives you've considered

I believe the current workaround is to lookup each video, find it's checksum, then find all matching checksums and other associated generated assets based on that checksum, then delete it, and re-run the "Generate" action again.

A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[Leopere]

Please use the issue templates it helps us keep organized.

[victorhooi]

Aha - I just took out the headings from the template - but added them back in, and the current suggested workaround (still not sure how this works for clips).

[bnkai]

related to #376

[WithoutPants]

TODO: add operations for regenerating sprites, previews, markers and transcodes.