[VM] Always save position within folder

[Yeechang Lee]

I am on 8.2.0b. If I visit a folder and change the displayed message,
VM doesn't remember the change unless I force a save (typically by
changing a message attribute, then reversing it). Can the .idx file
remember the position? Or another method?

I realize that not forcing a save of the entire folder just to alter
X-VM-Bookmark is usually a good thing, but that might be necessary. If
so, is there a way to auto-force vm-save-folder even if the folder is
otherwise unchanged?