feature: implement ewmh desktop viewport property

Set and update the _NET_DESKTOP_VIEWPORT property

http://standards.freedesktop.org/wm-spec/wm-spec-latest.html#idm140146176862048

> _NET_DESKTOP_VIEWPORT x, y, CARDINAL[][2]/32
> Array of pairs of cardinals that define the top left corner of each
> desktop's viewport.

include/ewmh.h

@@ -24,6 +24,12 @@ void ewmh_update_current_desktop(void);
  */
 void ewmh_update_number_of_desktops(void);
 
+/**
+ * Updates _NET_DESKTOP_VIEWPORT, which is an array of pairs of cardinals that
+ * define the top left corner of each desktop's viewport.
+ */
+void ewmh_update_desktop_viewport(void);
+
 /**
  * Updates _NET_ACTIVE_WINDOW with the currently focused window.
  *