Fix system tray icon on X11.

Observe Qt::BypassWindowManagerHint in XCB and use it for the
tray. Remove the call to XSync(), which sometimes was causing
the window to appear in the wrong place for a short time.

Task-number: QTBUG-31762

Change-Id: I7b4fc5e42b8c8bb9d03b1575be99d6d1c727be37
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>