[dwm] New layout

Hello everybody !

First forgive my english because I am french.
I am a new user of dwm. I used wmii before (for about one year). I tried dwm
on my eeepc because it is the fastest wm and because the display can be
optimzed at best on little screens (800x480 on my eeepc). I enjoyed dwm very
much so I decided to put it in my "main" computer whose screen size is
bigger of course. More I used it and more I feel a lack. With wmii, I was
able to have several columns, several windows on each column, to change the
windows' heights and the columns' width, etc... But these features are not
in dwm. So I decided to write a patch for dwm 5.0.1.

MODKEY + SHIFT + ( LEFT | RIGHT ) = move current window to the end of the
previous / next column.
MODKEY + SHIFT + ( UP | DOWN ) = swap the current window with the previous /
next window in the same column.
MODKEY + BUTTON3 = Resize the current window.

I have tested it but not enough(I use it now all the time to find bugs). So
if you find any bug or have any critical remark or if you want to improve
it, add features, or change some please tell me.

Kind regards.
QUINTIN Guillaume