Add Snapper compability

[xhess]

Snapper does not create snapshots with the date in the name. The snapshots are only numbered. The date and some other information is stored in the info.xml next to the snapshot. This code reads the date from the info.xml file created by snapper. For this plugin to work with the snapper structure you have to use something like "/data/.snapshots/%snapshot%/snapshot" as Snapshot format (where %snapshot% = 1,2,3,...).

[icewind1991]

I'm hesitant to merge this due to the added overhead of checking the existence of the xml file for every snapshot. The app already suffers from performance when you have a lot of snapshots.

Maybe an assumption can be made that either all snapshots have the xml file or none, then we only need to check if it exists once for non snapper setups.