unbreak xinerama support from r1.41 for panning setups
revert previous
fix menu/client placement in panning setups; XineramaQueryScreens gives
us the width of the psuedo screen, but here we need the edge instead
(xmax/ymax); just re-use w/h here for now.
teach screen_find_xinerama() about gap and adjust (simplify) callers;
menu becomes gap-aware for free.
add per-group vert/horiz tiling support; introduces 2 new bind commands,
'vtile' and 'htile'; from Alexander Polakov.
set the initial group to '1', missed by recent off-by-one group
numbering re-work; discovered the hard way by sthen@.

ok sthen@
put back r1.68 which allows an empty group to be sticky; behavior
change noticed by Thomas Pfaff and diagnosis why we need to
group_setactive in this case by Alexander Polakov.  replace XXX with
a useful comment.