Use RTF default color as default Qt format

Summary:
RTF spec says a color entry without anything defined is a "default
color", but doesn't specify more what that is. In practice seems to
work nicer now when such a default color is e.g. used to clear a
previously set text background color. Earlier version resulted in
black background and black text.

Reviewers: davidllewellynjones, dcaliste

Reviewed By: davidllewellynjones

Subscribers: Calligra-Devel-list

Tags: #calligra:_3.0

Differential Revision: https://phabricator.kde.org/D25715