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/formeditorplugin.cpp

@@ -105,7 +105,7 @@ bool FormEditorPlugin::initialize(const QStringList &arguments, QString *error)
     // Ensure that loading designer translations is done before FormEditorW is instantiated
     const QString locale = ICore::userInterfaceLanguage();
     if (!locale.isEmpty()) {
-        QTranslator *qtr = new QTranslator(this);
+        auto qtr = new QTranslator(this);
         const QString &creatorTrPath = ICore::resourcePath() + "/translations";
         const QString &qtTrPath = QLibraryInfo::location(QLibraryInfo::TranslationsPath);
         const QString &trFile = "designer_" + locale;