Tie everything together - track GMounts, an IMPORT mode, and copy the files & add them to a collection
This wraps up #29 (closed).
SourceManager is now tracking and notifying about GMounts representing USB mass storage and PTP devices.
There's code behind a GAction to copy the selected files out of the device to the host system, and add them to the chosen collection.
A new IMPORT mode to represent the import view in our state machine.