Organize screenshots

[nijel]

Describe the problem

There can be plenty of screenshots and there should be some way to organize them.

Describe the solution you would like

Allow to add user-defined topic to screenshots.

• Add topic management view.
• Add topic selection while creating/editing screenshot.
• Add filtering based on topic to the screenshot listing.
• Show topic when showing a screenshot.

Describe alternatives you have considered

Topic might not be the best name, but “label” and “category” are already used elsewhere in Weblate.

Screenshots

No response

Additional context

No response

[nijel]

@buhtz originally wrote at #11978:

Make it possible to somehow tag or group screenshots somehow. Searching for specific screenshots would be easier if I would have tags/groups like: "message boxes", "widgets", "Settings dialog" (a dialog having multiple screenshots because of tabs), "warnings and errors", "misc", "animated" (or by filetype png, gif, ...).