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/statsgrid.h

@@ -7,6 +7,7 @@
 #include <vector>
 
 class StatsAxis;
+class StatsTheme;
 class StatsView;
 class ChartLineItem;
 
@@ -16,6 +17,7 @@ public:
 	void updatePositions();
 private:
 	StatsView &view;
+	const StatsTheme &theme; // Initialized once in constructor.
 	const StatsAxis &xAxis, &yAxis;
 	std::vector<ChartItemPtr<ChartLineItem>> lines;
 };