fdo79368 Don't process overflow menu items twice.

We reuse the toolbox overflow menu for toolbarmanager's context
menu -- toolbarmanger previously added its menu listener to the
toolboxes menu permanently, meaning that it would try to handle
overflow menu items (in addition to the context menu items which
it should handle), instead we should only add the listener when
we are actually using the menu as a context menu.

Perhaps it would be better in the long run to actually use fully separate
menus instead, and ask toolbox to specifically add its items to that
rather than trying to hack the context menu on top of the overflow menu?

Change-Id: Iecface2c6eae9ab79dbcdb25ffdbaf446e2885ea
(cherry picked from commit 1ae89d189200c1e351a396a3f02612b84a69985f)
Reviewed-on: https://gerrit.libreoffice.org/9602
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>