Don't use stylesheets anymore for QTabWidget.

When using stylesheets there, all children (i.e. the QWebView) use
QStyleSheetStyle as well, which means the bug at [1] will annoy us.

We had to write our own font parsing as we can't use stylesheets, but at least
the bug is gone...

https://bugreports.qt-project.org/browse/QTBUG-40263