contextmenu & viewproperties: add special view and menu behavior for snapshots

I have been working on integrating Btrfs snapshots into KDE: https://invent.kde.org/system/kio-snapshot which among other things has new KIO workers to browse snapshots for subvolumes, and view snapshotted versions of a file.

Like those already present for the recentlyused or trash KIO workers, this MR introduces a special set of default view settings for the snapshot protocol. And for the snapshots-for-file case, it adds the open parent folder actions, same as recentlyused et al.

See #82

Part of kde-linux/kde-linux#666