Update the trash/delete action for the separate reader window

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