If a message which is in a trash folder is opened, the menu/toolbar
action would show "Move to trash", even though performing the action
would delete the message permanently. Correctly reflect the action in
this case.
New function KMail::Util::setActionTrashOrDelete() to modify the action,
copied from KMMainWidget::updateFolderMenu().
Differential Revision: https://phabricator.kde.org/D11284