Provide access to system configured fonts

Provide a "meta-font" that corresponds to the system configured font. This font can be streamed in/out etc. but will always result in the font that is configured in the windowing system.

Example:

QFont f = QFont::systemFont(QFont::StandardFont); // f is now Helvetica
// user re-configures his system
qDebug() << f;
// f is now Verdana