#37 Add support for removable media

[Anders Sandvig]

Monitor when removable media (typically USB drives) come online or go offline. When a drive's state changes, ask the user to (re)scan the drive if any folders in the library are located on this drive. When a drive goes offline, ask the user if the songs on this drive should be marked as "offline". Both feature should be possible to automate via options (i.e. automatically (re)scan and/or mark files as online/offline when a drive's status changes).

Songs located on offline media should be rendered in a darker color in the playlist?