Organize screenshots

### 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_