Add 'Add Separator' button to the 'Customize' dialog's
'Menus', 'Context Menus', and 'Toolbars' tabs. And remove the
'Add Separator' item from the 'Modify' menu.

In the following commits, more widgets will be added
and moved around to achieve the suggested appearance
and functionality in the related bug report.

Change-Id: I1961e2ebeb326eca69d8f967b2b636793200aba0
Reviewed-on: https://gerrit.libreoffice.org/28681Tested-by: Jenkins <ci@libreoffice.org>
Tested-by: Yousuf Philips <philipz85@hotmail.com>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>