CppEditor: Make value vs const ref return type configurable

... in the quickfix settings.
Also change the default: With mandatory RVO and move semantics, value
return types are the right choice most of the time.

Fixes: QTCREATORBUG-25790
Change-Id: I68fc6c616358478ba893101b3e04aa6bbbe79348
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/plugins/cppeditor/cppquickfixsettings.h

@@ -129,6 +129,7 @@ public:
     MissingNamespaceHandling cppFileNamespaceHandling = MissingNamespaceHandling::CreateMissing;
     QString memberVariableNameTemplate = "m_<name>";
     QStringList valueTypes; // if contains use value. Ignores namespaces and template parameters
+    bool returnByConstRef = false;
     std::vector<CustomTemplate> customTemplates;
 };
 } // namespace CppEditor