Implement EWMH desktop names

Implement EWMH desktop names

Maintain the _NET_DESKTOP_NAMES property on the root window.

http://standards.freedesktop.org/wm-spec/latest/ar01s03.html#idm140251368131760

> _NET_DESKTOP_NAMES
>
> _NET_DESKTOP_NAMES, UTF8_STRING[]
>
> The names of all virtual desktops. This is a list of NULL-terminated
> strings in UTF-8 encoding [UTF8]. This property MAY be changed by a
> Pager or the Window Manager at any time.