Allow testing wxGenericProgressDialog in the dialogs sample too

Add the possibility to test the generic implementation of the class when
we use the native one by default, this is useful to allow comparing the
behaviour of the two classes.

samples/dialogs/dialogs.h

@@ -452,6 +452,10 @@ public:
 
 #if wxUSE_PROGRESSDLG
     void ShowProgress(wxCommandEvent& event);
+#ifdef wxHAS_NATIVE_PROGRESSDIALOG
+    void ShowProgressGeneric(wxCommandEvent& event);
+#endif // wxHAS_NATIVE_PROGRESSDIALOG
+    void DoShowProgress(wxGenericProgressDialog& dialog);
 #endif // wxUSE_PROGRESSDLG
     void ShowAppProgress(wxCommandEvent& event);
 
@@ -596,6 +600,7 @@ enum
     DIALOGS_ONTOP,
     DIALOGS_MODELESS_BTN,
     DIALOGS_PROGRESS,
+    DIALOGS_PROGRESS_GENERIC,
     DIALOGS_APP_PROGRESS,
     DIALOGS_ABOUTDLG_SIMPLE,
     DIALOGS_ABOUTDLG_FANCY,