Qt Designer: Fix some clang warnings

- Use using instead of typedef
- User member initialization
- Use nullptr
- Do not use else after return
- Delete pointers unconditionally
- Fix some integer conversion warnings
- Use auto for new/casts to avoid type name repetition
- Use = default for trivial destructors

Task-number: QTCREATORBUG-23248
Change-Id: I0a7465d3aa200b5c862bec82636d2d22ddf8297b
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/designer/formtemplatewizardpage.cpp

@@ -60,8 +60,7 @@ Utils::WizardPage *FormPageFactory::create(ProjectExplorer::JsonWizard *wizard,
 
     QTC_ASSERT(canCreate(typeId), return nullptr);
 
-    FormTemplateWizardPage *page = new FormTemplateWizardPage;
-    return page;
+    return new FormTemplateWizardPage;
 }
 
 bool FormPageFactory::validateData(Core::Id typeId, const QVariant &data, QString *errorMessage)