Fix click to focus across screens when follow_mouse_focus is off

When follow_mouse_focus is set to False, clicking a window on another screen makes the border of the clicked window blue, but does not move the keyboard focus. Clicking a window on the same screen works as expected.

This is related to issue #125, although it now seems to happen only across screens: https://github.com/qtile/qtile/issues/125

This patch fixes the problem for applications that register _NET_WM_USER_TIME. This means most applications will work, but xterm will not. As suggested in issue #125 by tych0 I have attempted to get this working using ButtonRelease instead, but using the exact same code in a handle_ButtonRelease function produced the same symptoms as I was originally seeing.

I suggest merging this until a better solution can be found, to make click to focus usable for more than one screen.