Improve extra controls support in wxGenericFileDialog

Update the extra controls whenever anything changes in the dialog to
make the example in the dialogs sample actually work with the generic
dialog (previously nothing was updated at all).

Also delete the extra controls earlier to make the behaviour more
consistent with the native dialogs.

Use the new customization API for the generic dialogs in the sample too.

samples/dialogs/dialogs.cpp

@@ -1680,7 +1680,10 @@ static wxWindow* createMyExtraPanel(wxWindow *parent)
 class MyCustomizeHook : public wxFileDialogCustomizeHook
 {
 public:
-    explicit MyCustomizeHook(wxFileDialog& dialog)
+    // Normally we would just use wxFileDialog, but this sample allows using
+    // both the real wxFileDialog and wxGenericFileDialog, so allow passing
+    // either of them here.
+    explicit MyCustomizeHook(wxFileDialogBase& dialog)
         : m_dialog(&dialog)
     {
     }
@@ -1733,7 +1736,7 @@ private:
                      wxOK | wxICON_INFORMATION, m_dialog);
     }
 
-    wxFileDialog* const m_dialog;
+    wxFileDialogBase* const m_dialog;
 
     wxFileDialogButton* m_btn;
     wxFileDialogCheckBox* m_cb;
@@ -2005,7 +2008,8 @@ void MyFrame::FileOpenGeneric(wxCommandEvent& WXUNUSED(event) )
                     "C++ files (*.cpp;*.h)|*.cpp;*.h"
                  );
 
-    dialog.SetExtraControlCreator(&createMyExtraPanel);
+    MyCustomizeHook myCustomizer(dialog);
+    dialog.SetCustomizeHook(myCustomizer);
     dialog.SetDirectory(wxGetHomeDir());
 
     if (dialog.ShowModal() == wxID_OK)
@@ -2013,10 +2017,12 @@ void MyFrame::FileOpenGeneric(wxCommandEvent& WXUNUSED(event) )
         wxString info;
         info.Printf("Full file name: %s\n"
                     "Path: %s\n"
-                    "Name: %s",
+                    "Name: %s\n"
+                    "Custom window: %s",
                     dialog.GetPath(),
                     dialog.GetDirectory(),
-                    dialog.GetFilename());
+                    dialog.GetFilename(),
+                    myCustomizer.GetInfo());
         wxMessageDialog dialog2(this, info, "Selected file");
         dialog2.ShowModal();
     }