Don't block the import UI by waiting for a complete folder listing for the whole device, but build the model gradually.

The new flow goes like this:
1. When a connection to a camera is made, call methods for loading files and subfolders for the root folder.
2. The backend reports via signal about the subfolders in given folder.
3. Request is being made to download file listing for the returned subfolders, and then check for their subfolders.
4. Repeat until finished.


FIXED-IN:4.0.0