Don't change wxTextCtrl colour automatically under macOS

When wxColourDialog is shown, any changes to the colour selected in it
apparently result in broadcast messages to all currently visible text
controls, which was unexpected.

Block the changeColor: message to prevent this from happening.

Closes https://github.com/wxWidgets/wxWidgets/pull/830

include/wx/osx/cocoa/private.h

@@ -379,6 +379,7 @@ public:
     }
 
     - (void)textDidChange:(NSNotification *)aNotification;
+    - (void)changeColor:(id)sender;
 
     @end