[Bug]: Changing the Autoscan path should verify that the new path is empty

Currently, when changing the Autoscan path, any files in the new path will be copied to the `pending` folder. However, this is bug prone due to the lack of support for importing entire folders (see #33).

Expected behavior: when a new Autoscan path is chosen, verify that it is either empty or _only_ contains files (no folders).