ui: add support for notes on backup groups

ui: add support for notes on backup groups

Currently done a little bit hacky in a seperate API call following the
initial list_snapshots, as we previously didn't call list_groups at all
and instead calculated the groups from the snapshots.

This calls it async and updates the view with group comments when data
arrives. The editor is simply reused with the 'group-notes' API call,
since the semantics are the same.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>