TANGO: Add designerdialog icon from tango_testing and reference from human

Change-Id: I662842eb293d4b0d98ea2a39c3fd66abe1fc730b
Reviewed-on: https://gerrit.libreoffice.org/12217
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>