File tree component

[Gaurav0]

This PR does the following:

1. Adds a file-tree component
2. Hides it by default
3. Gives it the functionality of opening files in the active file editor panel
4. Creates a more complete full screen mode, rather than just numColumns=0
5. Hides the file-tree in that full screen mode
6. Adds a way to exit full screen mode

[Gaurav0]

@joostdevries @rwjblue @stefanpenner This PR is now complete. Please review.