feat: only poll for new status bar information in focused window

The main motivation for this MR is that the status bar makes multiple (2-4) requests every 30 seconds from every single VS Code window that is opened on user's machine. With a small condition, we can prevent 90% (my assumption) API calls made by the extension without degrading the user experience.

This MR also fixes the four-times upvoted #71 (closed).

This MR significantly simplifies and improves the status_bar code. It's done as a series of refactorings which are separated into commits. Each commit has a descriptive commit message explaining the change. The best way to review this MR is commit-by-commit with reading the commit messages.

Related to #212 (closed)