Monitor DBus for UDisks2.Filesystem signals and add file system path

The message received is very similar to a message sent by UDisks2 when
external media is mounted. The structure of the message must be

    string "org.freedesktop.UDisks2.Filesystem"
    array [
        dict entry(
            string "MountPoints"
            string "/media/KINGSTON"
        )
    ]
Fix compilation of tests
Improve adding file system paths

 - Check that path is not already added
 - Use name containing incremental index for external devices
Parse real UDisks2.Filesystem message