Add widget support for categories

### Describe the problem

Translating two different projects from one organization, for example:

Organization
- Project1
- Project2

In one Weblate project, you can use categories to separate translations
- Project/category1/component(s)
- Project/category2/component(s)

Currently, Weblate widgets work for all components in project or for a selected one.

It would be nice if will be possible to set a widget for a category to count statistics from all components in the category.

### Solution brainstorm

[![Translation status](https://hosted.weblate.org/widget/project/category/selected-widget.svg

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

_No response_