Add API to let applications integrate with the system fonts

This patch does 2 things mainly:
- Adds a QPlatformTheme font type for fixed fonts. It's important
because some OS provide specific monospaced fonts and we want to let
our applications to use the preferred fonts by default.
- Adds a new method and enum to QFontDatabase that expose the font
types that applications might need, so that they can make the
applications use the specific fonts that the system recommends. This
data was already available within Qt through the QPlatformTheme, but
it was not possible to use this data by Qt users. This new method
exposes such data.

Change-Id: Ic194c1e4bc07a70640672afd82ba756b87606985
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: David Faure (KDE) <faure@kde.org>