It was implemented on the xml handling side, but ignored
by the menubar code. In addition, make the menu:style
attribute survive customization (similar to tdf#114261).

Change-Id: I92a3517ee7ba2fe7a0782985d89fb5d0fdbac246
Reviewed-on: https://gerrit.libreoffice.org/46627Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Maxim Monastirsky <momonasmon@gmail.com>