Add wxColour::GetLuminance()

This method can be used to return the perceived brightness of the
colour.

Closes https://github.com/wxWidgets/wxWidgets/pull/1300

interface/wx/colour.h

@@ -158,6 +158,19 @@ public:
     wxUint32 GetRGBA() const;
     //@}
 
+    /**
+        Return the perceived brightness of the colour.
+
+        This value is computed using the simple @code 0.299*R + 0.587*G +
+        0.114*B @endcode formula with the coefficients taken from the RGB to
+        YIQ conversion formula and @c R, @c G and @c B being the values of the
+        corresponding colour channels normalized to 0..1 range, so that the
+        return value is 0 for black and 1 for white.
+
+        @since 3.1.3
+     */
+    double GetLuminance() const;
+
     /**
         Returns a pixel value which is platform-dependent.
         On Windows, a COLORREF is returned.