Fix for :DOCX-Preservation of Line style Dash type of "Shape with text inside"

    Issue :
        File containing Shape with text inside it having Line style as
        Dash type is not getting preserved after RT.

    Implementation :
        1] Added XML element <a:prstDash> with attribute  val="dash".
        2] Written Export Unit test case.

Conflicts:
	sw/qa/extras/ooxmlexport/ooxmlexport.cxx
Reviewed on:
	https://gerrit.libreoffice.org/7572

Change-Id: I27ca9f8786426a224b32dbd2aeabc40be244c9db