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/statsseries.h

@@ -10,6 +10,7 @@
 #include <QPointF>
 
 class StatsAxis;
+class StatsTheme;
 class StatsView;
 struct dive;
 class QRectF;
@@ -30,6 +31,7 @@ public:
 
 protected:
 	StatsView &view;
+	const StatsTheme &theme;		// Theme is only set once in the constructor
 	StatsAxis *xAxis, *yAxis;		// May be zero for charts without axes (pie charts).
 	QPointF toScreen(QPointF p);
 };