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/statsaxis.h

@@ -37,6 +37,7 @@ public:
 protected:
 	StatsAxis(StatsView &view, const QString &title, bool horizontal, bool labelsBetweenTicks);
 
+	const StatsTheme &theme; // Initialized once in constructor.
 	ChartItemPtr<ChartLineItem> line;
 	QString title;
 	double titleWidth;