Make the Add New function of the folder browser work a bit better

1. Ignore 404 errors from Dropbox and just display an empty list. Missing folders shouldn't worry us because we will create them when we push the todo file. This fixes #426, where the user attempted to select his folder without first having created the default /todo folder.

2. If you add a new folder, then go back into the folder browser, it now displays the folder in the correct hierarchy so that you can revert your choice by going back up the tree, etc. Again, it doesn't care that the folder(s) don't yet exist.