Fix Gui app wizard to write form classes consistent with Designer.

Make Gui app wizard use Qt Designer wizard code to write the classes.
Remove the settings from the form class wizard page extension area and
add a settings page instead. Polish the designer plugin's structs to
parametrize class generation with QSharedData and settings code.
Export them and use them in the Gui app wizard.

Task-number: 256927

src/plugins/designer/settingspage.cpp

@@ -28,9 +28,11 @@
 **************************************************************************/
 
 #include "settingspage.h"
+#include "designerconstants.h"
 
 #include <extensionsystem/pluginmanager.h>
 #include <qt_private/abstractoptionspage_p.h>
+#include <QtCore/QCoreApplication>
 
 using namespace Designer::Internal;
 
@@ -55,12 +57,12 @@ QString SettingsPage::trName() const
 
 QString SettingsPage::category() const
 {
-    return QLatin1String("Designer");
+    return QLatin1String(Designer::Constants::SETTINGS_CATEGORY);
 }
 
 QString SettingsPage::trCategory() const
 {
-    return tr("Designer");
+    return QCoreApplication::translate("Designer", Designer::Constants::SETTINGS_CATEGORY);
 }
 
 QWidget *SettingsPage::createPage(QWidget *parent)