Also sort the notes based on when last modified. Now that we're moving
towards being a more note-taking app, it makes sense that a recently
modified note goes on top.
Of course, the sorting should be controllable by the user, but the
default probably should be by when modified.
Now when adding/editing/removing a note, it gets modified from the
directory it was present in. There is no longer a just a plain list of
all notes, but always a tree of notes, which are inside Folders.
