Add QProtectedSignal

With https://codereview.qt-project.org/c/qt/qtbase/+/436854 implicit conversions from initializer_list were disabled for QPrivateSignal.

This change makes a lot of sense, still it went too far as it prevents perfectly legit signal emission from subclasses implementing an abstract base class.

To work around one could extend the Q_OBJECT macro to also define QProtectedSignal, which is exactly the same as QPrivateSignal, but with protected visibility instead of private visibility.

Thank you.

PS: How comes that this significant source compability break doesn't have an Jira ticket assigned. This doesn't only violate Qt Project policy, it also prevents that this significant source compability break is listed in the release notes.