Finish the desktop GUI prototype.

TODO:

- [ ] the up arrow does not work as expected:
    - [ ] expected: `1.0, 2.1` `->` `1.0, 1.1`, actual: no change
    - [ ] expected: `1.2, 1.3` -> swap, actual: no change
- [ ] add the ability to click on items in a document
- [ ] add threading to load the tree
- [ ] add scrollbars
- [ ] add icon