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.

src/main.c

@@ -674,6 +674,7 @@ int main(int argc, char *argv[]) {
     /* Set the ewmh desktop properties. */
     ewmh_update_current_desktop();
     ewmh_update_number_of_desktops();
+    ewmh_update_desktop_viewport();
 
     struct ev_io *xcb_watcher = scalloc(sizeof(struct ev_io));
     xcb_check = scalloc(sizeof(struct ev_check));