QSystemTrayIcon::activated is called more than once per click

This happens for instance with the StatusNotifier based tray icon backend in Qt 5.5.0. It should affect all qpa-based tray implementations.

To reproduce:

• Create a QSystemTrayIcon
• Connect the activated signal
• show(), hide(), show()
• Click the tray icon
• The slot will be invoked twice.

The signal will be called once for each pair of hide/show calls.

The bug seems to be in install_sys_qpa: the signal is connected each time show() is called. I have a patch.