Allow keeping media on camera after it's dumped

[klausweiss]

At the time of writing the program dumps media from the camera and deletes it from the device after a successful download. Some users have raised interest in the program not deleting the media afterwards. I think it's a valid thing to expect from this software.

Allow keeping the media on the device after it was dumped.

Requirements:

• a new config flag added for controlling whether to delete media from device upon downloading (delete by default),
• the logic implemented according to the flag above,
• the implementation can be considered clean code.

Considerations:

• the software doesn't do much at this point, hence manual testing is straightforward an not time-consuming. This change would add conditional logic to the program, so we should introduce some sort of automated testing. Unit tests will do for now.
• When all photos from a camera are dumped, and camera reconnects with these same photos, they'll be downloaded again. This is a waste of time (and battery). Let's consider interrupting file download early, if a file with the same name was already discovered on disk. I definitely wouldn't want files to be deleted from camera if the download was skipped (risk of losing data), so it's worth considering if this should only be the case for the "keep media" mode.