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

[RobRoyce]

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).