Some clients - such as Element - can send colored text through <span>,
which fails to display in Qt's rich text parser. So we need to transform
that into CSS styles which is supported by Qt.
Notably this allows you to exchange rainbow shrugs through Matrix, which
is really important. And this means colored backgrounds for text is
supported too, I guess.
(cherry picked from commit 843da2664f)
a046e3ed27