Display total space used by all snapshots

[tbertels]

Is your feature request related to a problem? Please describe.
When btrfs quotas (qgroups) are enabled with btrfs quota enable <mnt>, Timeshift displays the space used by each snapshot.
If we want to know how much space we could potentially free up by removing multiple snapshots, we have to add all values.

Describe the solution you'd like
Timeshift could display the total space used by all snapshots on the bottom bar, between the number of snapshots and the free space.

Describe alternatives you've considered
Display the total space used by the selected snapshots.

Additional context

[github-actions]

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[tbertels]

Still not implemented.