Add highlighting usage

Close #9.

This PR isn't ready for merging. I open it early for discussion.

Highlighting usage currently works for `*.fs` files in an active project.
A few things need to be done:
 - [x] Add non-blocking GUI
 - [x] Refactor `HighlightUsageTagger` to be more readable
 - [x] Improve F# Project System
 - [x] Add an option to disable the feature
 - [x] Fix bugs and consistencies (I noticed some but didn't investigate them further)

However, the biggest issue is to access F# Project System. I did implement by trial and error. I managed to get references and source files, but couldn't get compiler arguments properly. It would be nice to understand F# Project System (it has no documentation at all) and update the implementation.