Added constructor function for OPropertyBag in /comphelper/source/property/.

Change-Id: I28d6aa5c6fa2aa4324cf4ac61c5f2e3ab0a5e14a
Reviewed-on: https://gerrit.libreoffice.org/23541Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>