Support for exclusions and external drives in the BTRFS mode

[Anderhar]

The BTRFS mode of Timeshift is known for its inability to exclude unwanted folders from the snapshots, as well as the inability to work with external drives (i.e. it is not a backup solution technically). However, the BTRFS itself provides ways to implement both of these features like this e.g.:

• Include/Exclude menu may be added in the form of a blacklist, replenished via automated conversion of chosen directories to the BTRFS subvolumes, and backward conversion of subvolumes to directories when removed from the blacklist. Today, some users are already accomplishing this task with a simple sequence of CLI commands that can be easily wrapped into a single GUI button.
• Backup to an external drive is also already implemented in some CLI tools via btrfs send and btrfs receive commands, but unfortunately nobody has made a GUI for it yet.

And since all of this seems pretty obvious and simple, then it would be nice to at least get comments on the implementability of this features in Timeshift, because without them it feels a bit incomplete.