Fix fdo#84591 Styles dropdown disappears when clicked on submenu

This tells the dropdown to not grab the focus.

Change-Id: If1b32d1f53c12eb86ab2f92c531385578d82ae59
Reviewed-on: https://gerrit.libreoffice.org/11841
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>