QXcbWindow::setAlertState sets property instead of sending client message

According to EWMH a client should send a client message in order to change the state. Verbatim quote:
"To change the state of a mapped window, a Client MUST send a _NET_WM_STATE client message to the root window"

But QXcbWindow::setAlertState calls QXcbWindow::setNetWmStates which is otherwise only used to update the states prior to mapping. Thus setNetWmStates updates the property instead of sending a client message.

By not sending a client message a strict EWMH compliant window manager doesn't know that the window is demanding attention. E.g. KWin provides a global shortcut (ctrl+alt+a) to activate the window demanding attention. While the taskbar which only reads the state shows the window demanding attention, the shortcut doesn't work.

Attached is a small qml example which can illustrate the problem with KWin. Steps to reproduce:
1. qmlscene test.qml
2. The window shouldn't be focused but demands attention
3. Press ctrl+alt+a to focus
4. click the button
5. focus another window
6. wait 5 sec
7. notice that taskbar shows window as demanding attention
8. press ctrl+alt+a

Actual result:
nothing happens

Expected result:
window gets focus