xcb: don't select XInput events on the root window

If we select XInput events, then when the mouse is clicked, there will
not be a fallback to a core pointer event. But a typical Qt application
doesn't own the root window.  If the window manager (such as OpenBox,
Awesome or fvwm) relies on receiving core pointer click events, e.g.
to show a desktop menu, then each time a device is hotplugged while a
Qt application is running, we would select XI2 events and thereby
prevent the window manager from receiving them.

QDesktopWidget's native window is added to m_mapper, even when
it isn't mapped.  Then after hotplugging there's a hierarchy event,
and that calls xi2Select for every window in m_mapper.  The assumption
with this patch is that the root window does need to be in m_mapper
in case the QDesktopWidget is shown (that was done already in Qt 5.1: 
fca94fa5ed8321e84e7b0ff515620fbb901db545), but xi2Select must avoid 
selecting XI2 events on it to fix this bug.

Task-number: QTBUG-49952
Change-Id: I5c160e879d93fadfce14120ef2e89a4f71d4f599
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>

src/plugins/platforms/xcb/qxcbconnection_xi2.cpp

@@ -274,7 +274,7 @@ void QXcbConnection::finalizeXInput2()
 
 void QXcbConnection::xi2Select(xcb_window_t window)
 {
-    if (!m_xi2Enabled)
+    if (!m_xi2Enabled || window == rootWindow())
         return;
 
     Display *xDisplay = static_cast<Display *>(m_xlib_display);