DocxAttributeOutput::FormatBackground: handle dml syntax

CppunitTest_sw_ooxmlexport's testTextFrameBorders is a reproducer for
this problem.

Change-Id: I52e93ff09c67222caec789fc93bc19c257cdee05
DocxAttributeOutput::WriteDMLTextFrame: write effect list

We already had code for this, but only for pictures, factor out the
relevant code into its own method and call it here as well.

CppunitTest_sw_ooxmlexport's testTextFrameBorders is a reproducer for
this problem.

Change-Id: I3b71e64512a65c42a2f3859b4912a4408944e697