statistics: collect colors in a StatsTheme class

To enable rudimentary theming, collect all colors in a new
theme class. The class has to be passed down to the various
items.

In general the items save a reference to the them in the
constructor. Alternatively, they might also just query
the StatsView everytime they need to access a color.
For now, it's hard the say what is preferred: a reference
per item or a function call per invokation?

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

stats/informationbox.h

@@ -20,6 +20,7 @@ struct InformationBox : ChartRectItem {
 	void setPos(QPointF pos);
 	int recommendedMaxLines() const;
 private:
+	const StatsTheme &theme; // Set once in constructor.
 	QFont font; // For future specialization.
 	double width, height;
 };