fdo#65295 - add export of text-box gradient fill angle

Currently - the angle of the gradient fill of a text-box was not
being exported. This resulted in the gradient always being a horizontal
gradient. This patch fixes that and exports the angle.

Conflicts:
	sw/qa/extras/ooxmlexport/ooxmlexport.cxx
Reviewed on:
	https://gerrit.libreoffice.org/6653

Change-Id: I1da60a8e82705097e9e699549d3828375967e744