add boolean argument to set_window_{above,below}

There may be another way to do this that I've entirely missed. But to me, it looks like if a window spawns itself as above, there's no way using devilspie to remove that flag.

I'm using it to put Flash back on the normal layer after making it fullscreen in Openbox, which apparently doesn't detect the creation of the plugin_container, possibly because it technically already exists whenever plugin content exists in Firefox.