[dwm] clients per tag patch

I've written a patch for dwm to limit the number of clients per tag
to be displayed, usually you work with 2 or 3 windows at the same time,
no more, the rest are less important, so you can switch between (all or 2)
windows per tag with a keybinding.

        { MODKEY, XK_w, clientspertag, "0" }, \
        { MODKEY|ShiftMask, XK_w, clientspertag, "2" }, \

The patch is really simple and not really intrusive, the only annoying thing
is that you can walk over non-visible clients on the slave area, but this can be
a feature if you know what windows are there allowing you to switch between
N+1 windows easily.

clientspertag("0"); -> show all windows
clientspertag("2"); -> shows only two windows

arg what do you think about adding this to dwm?

another possible implementation would implementing the clientspertag() function
as a toggable option, so you will not need to press two different keys for this
and defining the clients_per_tag value as #define in config.h

  --pancake