Exclude mount points to other filesystems like rsync's --one-file-system

[StanislavNikolov]

Is your feature request related to a problem? Please describe.
Currently it looks like Vorta follows mounts to other file systems, which is undesirable in my case, especially for sshfs through FUSE.

Describe the solution you'd like
I would like to have an option to not cross filesystem boundary, not just by listing mount points in exclude patterns. Rsync's --one-file-system does the same thing.

Describe alternatives you've considered
It is also possible to make a pre-backup command to unmount everything inside the to-be-backed-up path, but that will mess up my workflow if I request a backup while I work.

[m3nu]

Already possible with #677, but not ready to merge yet.