Don't let the OS maintain relative Z-order for windows

My reasoning is the following: GTK window-handling logic is mostly derived from X, and, AFAIK, X WMs do not try to keep relative Z-order intact when one of the windows is raised or lowered. Therefore we should just disregard this functionality that Windows WM has, and change window Z-order without regarding any other window that might be below or above it. We do have functions that ensure correct stacking order, just for these cases.