Fix windows that are maximized on startup

It seems that setting the xprop on an already maximized window doesn't
actually take effect immediately. By unmaximizing and remaximizing it
after setting the xprop, things work as expected.

This does mean that we can't use Util.spawn any longer, since it doesn't
return any reference to the spawned process. Instead, we mimic what
Util.spawn does internally, but without the extended error handling
(which isn't really needed anyway). This allows registering a completion
handler that remaximizes the window.

Closes: #7
I tried logging Util.MAXIMIZED, which gave empty log output (probably meaning it's undefined). Looking around the gnome-shell repository, it seems there is no MAXIMIZED variable defined (I checked both [util.js in master](https://git.gnome.org/browse/gnome-shell/tree/js/misc/util.js) as well as grep through _all_ changes in the entire repo for the literal string `MAXIMIZED`). Does it actually have a value for you?

Also, I don't suppose that a short blink on partly-maximized windows is really a problem, though it would be nicer to fix it I guess. We could of course hardcode the value 3 (which I think is the right one, from logging current values).