implement support for _NET_WM_DESKTOP properties on windows.

it works kinda like _CWM_GRP, which we added to aid restarts a while
ago, but it's standardised and clients are specifically allowed to set
it to request a desktop.

for noe we leave _CWM_GRP support in, but its days are now numbered.

while i'm here fixup an int/long mixup with an earlier diff.

ok okan@