Allow automatic OS-provided tabbing to be disabled in wxOSX

macOS 10.12+ implements automatic tabbing in the OS. This adds
entries to the menus and also adds a tab bar. Some applications
might want to disable this, so provide an interface for doing this.

Closes https://github.com/wxWidgets/wxWidgets/pull/1674

include/wx/osx/app.h

@@ -139,6 +139,9 @@ public:
     // override this to return false from a non-bundled console app in order to stay in background ...
     virtual bool         OSXIsGUIApplication() { return true; }
 
+    // Allow the user to disable the tab bar support in the application
+    void                 OSXEnableAutomaticTabbing(bool enable);
+
 #if wxOSX_USE_COCOA_OR_IPHONE
     // immediately before the native event loop launches
     virtual void         OSXOnWillFinishLaunching();