Make the "all" filter, URI to URN monitoring and SelectionToolbar more flexible, and a widget to display an attached device
Another batch of patches for #29 (closed).
GMount-backed sources representing attached devices shouldn't contribute towards the SourceManager's "all" filter because they don't represent a search criterion.
We need to find out the URN used to represent a newly imported GFile. Therefore, we can't "wait_for_changes" to a BaseItem, but need something that waits for a GFile.
We need a widget to show the attached devices.
The SelectionToolbar needs to be able to show and hide buttons, as opposed to just changing their sensitivity.