More rabidly comply with ICCM and EWMH

While we're hacking on that, make a first attempt at a generic
Atom access framework.

The only actual visible change that should occur here is that the
window will not resize larger than will fit on the current desktop
between various WM widgets (panels and menus).  I've been considering
this change for a while, but now that fullscreen support is present,
it doesn't really take anything away.