Create selector functions for root file, active file

[lindapaiste]

Continue breaking up stale PR #2054 into smaller pieces.

Changes:

• Create memoized selectors for deriving values from the Redux state:
  • The root folder
  • The file which is currently open in the editor
• Use those selectors in mapStateToProps and useSelector in existing components

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123