use similiar style for client flags
Introduce a check to see if a group holds only 'sticky' clients and use
this check to decide if a group is virtually empty.  Rationale: if a
group contains *only* 'sticky' clients, it should be skipped while
cycling through groups.  Apply similar logic to the group menu.

Based on an idea from phessler@, who also tested another version.
Implement EWMH _NET_WM_STATE_HIDDEN.
don't toggle _WM_STATE_HIDDEN here yet
Use a similarly named check as sticky for hidden check in a group.
ewmh states _NET_WM_STATE_STICKY should not alter position
these client actions are just toggles; less confusing with better names
Move motion time check to the top of each MotionNotify block (and
eliminate from ButtonRelease); further limits the amount of work done
outside the threshold, notably mousefunc_sweep_calc,
screen_find_xinerama and client_snapcalc.
Move stuff that doesn't belong in group_init; while here, explicitly
initialize hideall and cycling.
Update _NET_CURRENT_DESKTOP with the screen's group_active->num.