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.

include/ewmh.h

@@ -24,6 +24,12 @@ void ewmh_update_current_desktop(void);
  */
 void ewmh_update_number_of_desktops(void);
 
+/**
+ * Updates _NET_DESKTOP_NAMES: "The names of all virtual desktops. This is a
+ * list of NULL-terminated strings in UTF-8 encoding"
+ */
+void ewmh_update_desktop_names(void);
+
 /**
  * Updates _NET_DESKTOP_VIEWPORT, which is an array of pairs of cardinals that
  * define the top left corner of each desktop's viewport.