Add applicationDisplayNameChanged() signal to QGuiApplication

Add a QGuiApplication::applicationDisplayNameChanged() signal.

If applicationDisplayName is no set applicationDisplayNameChanged() is emitted after applicationNameChanged() via a signal to signal connection.

Once set the signals are disconnected and applicationDisplayNameChanged() gets emitted by setApplicationDisplayName().

Note: this would be a first step for allowing access to applicationDisplayName from QML using Qt.application.displayName.