This PR improves statistics counting, as well as adds support for more of them, fixing #63 (closed). It does so in a iA Writer inspired way, meaning:
- The stats bar displays one stat, word count by default.
- The stats bar now contains a button that displays all stats and allows toggling between them.
- The default stat is saved between sessions.
- All the stats are objective and deterministic. For instance, I contemplated adding GhostWriter's "Pages" estimation, but it's outright broken in my testing, and I also think we should be strict about what to include. Too many things will make it less useful. Regardless, it's trivial to extend.
- Calculations are done on a worker thread, to prevent hogging the UI and allowing future extensibility without much consideration.