Allow turning on/off opacity selector in wxColourDialog (wxOSX)

This feature to show/hide alpha values and opacity selector (slider) is
already implemented under wxGTK and for generic wxColourDialog.
For the sake of backward compatibility, this feature is enabled here by
default (through the corresponding property of wxColourData).

interface/wx/colourdata.h

@@ -46,6 +46,16 @@ public:
     */
     bool GetChooseFull() const;
 
+    /**
+        Indicates whether the colour dialog will display alpha values
+        and an opacity selector. It is meaningful under wxGTK, wxOSX
+        and with regards to generic colour dialog.
+
+        The default value is @false, except wxOSX where it is @true (for
+        the sake of backward compatibility).
+    */
+    bool GetChooseAlpha() const;
+
     /**
         Gets the current colour associated with the colour dialog.
 
@@ -72,6 +82,16 @@ public:
     */
     void SetChooseFull(bool flag);
 
+    /**
+        Tells the colour dialog to show alpha values and an opacity selector
+        (slider). Currently it has effect under wxGTK, wxOSX and for generic
+        colour dialog.
+
+        The default value is @false, except wxOSX where it is @true for
+        backward compatibility.
+    */
+    void SetChooseAlpha(bool flag);
+
     /**
         Sets the default colour for the colour dialog.