[dwm] [patch] move clients in layout

Hello! This is my first message to this mailing list so I thought I'd start
by contributing a patch. This patch allows you to move clients around to
reposition them in a tiled layout. The behavior is meant to emulate
Xmonad's mod-shift-k and mod-shift-j. m-s-j swaps a client with the next
visible, tiled client in the clients list. m-s-k swaps the client with the
previous.

I hope some of you find this useful and I certainly hope to contribute more
in the future.

-Niki Yoshiuchi