Implements new "Batch ops" option menu in Message List. Provides for

flag/unflag, mark as read/unread and delete.  Also provides explicit
mode changing, and select/deselect all.  Move and copy are partially
implemented, but disabled.

Desperately needs a new icon.

The "Sort by..." menu now toggles ascending/descending when the
currently selected sort mode is clicked.

Also, an ineffective change to setting the title in the Message List.
A bit more work is in order on that.