ui: color schemes

Put all colors into single xml file. Accessing colors is handled via the singleton class ColorScheme. The default scheme
"bright.xml" holds the original colors. The scheme "dark.xml" is a start, but still a poor alternative. Color schemes
can be switched via the View menu.

src/lib/component/view/GraphViewStyle.h

@@ -39,6 +39,7 @@ public:
 		NodeStyle();
 
 		std::string color;
+		std::string textColor;
 
 		std::string shadowColor;
 		int shadowBlurRadius;