Restore Q_OBJECT in ALL QObject-inherited classes

Issue #520 (closed) is also caused by lack of Q_OBJECT macro.

Presumably there are other sleeping bugs introduced by MR !1447 (merged).

Suggestion: completely restore Q_OBJECT macro in all classes inherited from QObject.

The check for explicit qobject_cast use (#521 (closed)) is not enough because similar casting can be done implicitly, inside Qt.