Provide way to create a snapshot tree with off-filesystem subvolumes

[CopOnTheRun]

One of the features of btrfs is the send/receive functionality which transports subvolumes to different filesytems. Currently btrview only works on a per filesystem basis, being able to create a snapshot tree across btrfs file systems could provide better clarity of where and how snapshots are stored.

Off filesystem snapshots fall into two categories:

• non-networked - other btrfs filesystems that are mounted somewhere on the filesystem. These should be relatively easy to deal with because I already have them when creating snapshot forests. The only thing that really needs to happen is they have to be grouped and fed through Btrfs.get_forest. The hard part will be figuring out how I want to cli output to show this.
• networked - snapshots that have been sent to other machines, or are connected via a networked filesystem. In theory as long as I can get the output of btrfs subvolume list and btrfs subvolume show on those filesystems I can create the tree, but this won't be super easy as there's probably 100 different ways to do that, all with their own quirks.