Ну, тогда вот так:
C++ (Qt)
#ifdef Q_WS_X11
Display *dpy = QX11Info::display();
Atom property = XInternAtom(dpy, "_NET_WM_STATE_SKIP_TASKBAR", False);
Atom net_wm_state = XInternAtom(dpy, "_NET_WM_STATE", False);
XChangeProperty(dpy, w, net_wm_state, XA_ATOM, 32, PropModeAppend,
(unsigned char *) &property, 1);
#endif