The old code checked that a device's parent was a USB drive. This works for UMS devices that ahve a partition, but not for others. Now we also check if the device itself is a USB drive.
- Basic copy to/from device support.
- When refreshing library/albums, only affect parts of the model that have
changed. (Previously the whole model was replaced).
- Allow songs to be deleted.
TODO:
- Make song deletion actually work
- Copy/delete album covers
- When deleting a whole album/artist, need to delete corresponding dir if it is empty.
- MTP devices
