a few problems here

*) each symbol was drawn on top of the previous one instead of clearing it
*) some spurious placeholder text in the menu entry
*) m_nSymbolType not set correctly on activation of a submenu entry
*) under gtk3 adding entries to the submenus didn't inform the
   menubutton of their addition
*) we can drop m_xMenu because set_item_sensitive can be used
   instead, which didn't exist at the initial time of writing

Change-Id: Id339992c4192f3782fddfd56cb3e9b67cfcbe2a2
Reviewed-on: https://gerrit.libreoffice.org/65977
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>