Implement quirk NOFOCUSCYCLE to remove a window from the normal focus cycle

If you assign "NOFOCUSCYCLE" to a window, the normal focus_next,
focus_prev will never select it. But have no fear m'am! You can still
focus the window though search_win and the mouse.

I use it for monitoring applications and for screen-capture programs such as "screenkey" or "keymon".

Maybe a better option would be to set the _SKIP_PAGER/_SKIP_TASKBAR atoms?
Comments appreciated.