QSystemTrayIcon: Fix double connection with qpa.

install_sys_qpa() is called for each QSystemTrayIcon::show(),
leading to spurious calls of user slots if the signal isn't
disconnected on remove_sys_qpa().

Task-number: QTBUG-48068
Change-Id: Ic830250c71fee6b584550ab5aabf54289e478fcb
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Richard Rondu <rondu.richard@gmail.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>

src/widgets/util/qsystemtrayicon.cpp

@@ -693,6 +693,10 @@ void QSystemTrayIconPrivate::install_sys_qpa()
 
 void QSystemTrayIconPrivate::remove_sys_qpa()
 {
+    QObject::disconnect(qpa_sys, SIGNAL(activated(QPlatformSystemTrayIcon::ActivationReason)),
+                        q_func(), SLOT(_q_emitActivated(QPlatformSystemTrayIcon::ActivationReason)));
+    QObject::disconnect(qpa_sys, &QPlatformSystemTrayIcon::messageClicked,
+                        q_func(), &QSystemTrayIcon::messageClicked);
     qpa_sys->cleanup();
 }