Quirk noborder

New quirk "NOBORDER", which disables the border when the window is
floating and unfocused. That is, the border disappears if the window is
not currently focused and floating, but comes back as soon the window is
focused. The border is always there if the window is tiled though.
I use it for monitoring applications, for which border is only waste
of space, and for screen-capture programs such as "screenkey" or "keymon".

I usually use this quirk together with NOFOCUSCYCLE in #78