Improve settings tree title and descriptions

- All settings are now phrased in the imperative form stating what the
  setting does rather than talking about what it controls. (E.g.:
  "Show `Debug` action." instead of "Whether to show `Debug` action"
- Categories are now displayed in title case
- Categories are now sorted lexicographically
- General category is removed (and all the settings are moved to the top
  level)
- Language for a few descriptions is made a bit less ambiguous

Author: joshka