If _NET_WM_DESKTOP is set to -1 during client creation, place the client into
group 0 (nogroup); solves problem initially discovered by oga@nicotinebsd with
tint2.  A clientmessage *after* client creation already handles this case.

Go further and assign every client to a group; in non-sticky mode, group 0
(nogroup) and sticky mode, the active group.  In both cases, autogroup will
override the group assignment.  Removing a group from a client always places
the client back into group 0 (nogroup).  Autogroup can also assign a client to
group 0 (nogroup) to keep a client always visible (unless of course one opts to
hide all clients).
all mapped clients now should have a group, so simplify some cases
Replace a few hand rolled loops with like tailq loops.
Make sure we cycle through existing client's group membership and set
the hidden state on the group; important to know group hidden state
after a re-exec or restart of cwm (as easily seen in group menu).