Use forwarding reference when passing function objects

So instead of passing every function objects by value (a copy is made), now function objects
will be handled properly by reference (for lvalues) or by value (for rvalues).

src/base/settingvalue.h

@@ -50,7 +50,7 @@ public:
     // T proxyFunc(const T &a);
     template <typename ProxyFunc>
     explicit CachedSettingValue(const char *keyName, const T &defaultValue
-                                , ProxyFunc proxyFunc)
+                                , ProxyFunc &&proxyFunc)
         : m_keyName(QLatin1String(keyName))
         , m_value(proxyFunc(loadValue(defaultValue)))
     {