add button to import a thumbnail manually on a video with the MediaBox

[Teo-conrath]

add button to import a thumbnail manually on a video with the MediaBox, only one file has been updated

What kind of change does this PR introduce?

Add a button

Why was this change needed?

Until now we must choose a frame of the video but sometimes it's difficult to find a good frame.

Please link to related issues when possible, and explain WHY you changed things, not WHAT you changed.

Other information:

eg: Did you discuss this change with anybody before working on it (not required, but can be a good idea for bigger changes). Any plans for the future, etc?

Checklist:

Put a "X" in the boxes below to indicate you have followed the checklist;

• I have read the [CONTRIBUTING] guide.
• I checked that there were not similar issues or PRs already open for this.
• This PR fixes just ONE issue (do not include multiple issues or types of change in the same PR) For example, don't try and fix a UI issue and include new dependencies in the same PR.

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