Add a more convenient wxColour::MakeDisabled() overload.

Allow creating a disabled version of the colour without having to manually
break it into RGB components and then recreating it from them.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72618 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/colour.h

@@ -167,6 +167,7 @@ public:
     static void          ChangeLightness(unsigned char* r, unsigned char* g, unsigned char* b, int ialpha);
 
     wxColour ChangeLightness(int ialpha) const;
+    wxColour& MakeDisabled(unsigned char brightness = 255);
 
     // old, deprecated
     // ---------------