Run command after successful snapshot creation

[Semnodime]

Is your feature request related to a problem? Please describe. I am using grub-btrfs to allow booting into recent snapshots. To update the list of available snapshots within grub, I need to manually run a command upon creation of a new snapshot.

Describe the solution you'd like A new text field within the misc tab of the timeshift wizard that allows to run an arbitrary command every time a new snapshot has been created by timeshift (both manual snapshot and planned).

Describe alternatives you've considered Integrating grub-btrfs directly into timeshift is not aligned with the idea to do one thing and do it great.

Additional context Useful for (and thus related to) #150