[dwm] Cycle urgent windows patch for dwm-4.8

Hi,

I've made a function for cycling through the windows with the urgent flag
set. The idea is that when first called, you start a cycle which on
completion will give focus back to the window which had it initially. This
window's prevtags are saved. Each call to focus() breaks the cycle, so
you don't have to complete it to continue using dwm as normal.

To be fair, I've only tested the patch with one urgent window. If you
have any ideas as to how to trigger the urgent flag manually,
please let me know.

Best regards,

Martin Oppegaard