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/regressionitem.h

@@ -5,6 +5,7 @@
 #include "chartitem.h"
 
 class StatsAxis;
+class StatsTheme;
 class StatsView;
 
 struct regression_data {
@@ -20,6 +21,7 @@ public:
 	void updatePosition();
 	void setFeatures(bool regression, bool confidence);
 private:
+	const StatsTheme &theme; // Initialized once in constructor
 	StatsAxis *xAxis, *yAxis;
 	regression_data reg;
 	bool regression, confidence;