attachment grid: move all-parts controls to tbar, transform back to collapsible title

attachment grid: move all-parts controls to tbar, transform back to collapsible title

just better ergonomics and I don't think a lot of people will care
about the parts list, especially if the raw email is already visible
anyway.

Also note that while yes, the content-disposition is user
controllable, most (all?) MUAs only show it if set so even
spammers/attackers have it in their interest to set it.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>