Add search/filter function

[humdingerb]

Sometimes, it'd be nice to be able to search/filter events after category or search terms. This makes ever more sense the larger the time span you're looking at. Not so much for "Day", but for "Month" or a to-be-added "Year" tab, it'd be nice.
There could be a "Category" popup and "Search terms" textview above the tab views like in HaikuDepot.

[humdingerb]

More info in the discussion at https://discuss.haiku-os.org/t/gsoc-22-project-improving-the-calendar-application/12139

One BTextControl that filters the currently viewed tab (day, week, month) and looks through all fields (category, description, location) should suffice.