Restore Category filtering functionality

[CharliePoole]

Until July, 2021, the GUI had a category filtering function, embodied in a separate CategoryView class. This was eliminated by PR #736.

At the time, I thought that the ability to display tests by category and run the tests in a given category would provide sufficient functionality to replace the CategoryView. However, running all the tests in one category is only the simplest category-based operation. Even though the engine supports arbitrary filters (the where clause in the console runner), the GUI has no way for the user to specify such a filter.

An equivalent of the old CategoryView is needed. Design is required. It may be a separate view or just a tool window. It should allow simple inclusion and exclusion but also permit the entry of where clauses of arbitrary complexity. Further, when a filter has been set, the tree should indicate clearly which tests are included and which are excluded.

Obviously, this is a major new feature, which may need to be implemented incrementally.