Remaining tasks after categorization PR

These were the leftover tasks from #145

- [ ] Merge web watcher events into window events in the query? (to allow for classifying by url/domain)
- [x] ~~Category Tree: If a category has children, but also activity attributed directly to the parent that does not belong to a child, then create a "Other" child containing the activity.~~
- [ ] Should we have a way to filter on a specific event key? Right now the regexes are applied on every (string) data value. (#286)
- [x] Add a way to show how/which events are (un)categorized
- [x] Fix summary view to also use Vuex and all the new goodies
- [ ] Improve default categories (#174)
- [ ] Link to docs and/or regex101, explaining how regular expressions work and how to write good categorization rules.