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.

src/commands.c

@@ -1987,6 +1987,9 @@ void cmd_rename_workspace(I3_CMD, char *old_name, char *new_name) {
     ysuccess(true);
 
     ipc_send_event("workspace", I3_IPC_EVENT_WORKSPACE, "{\"change\":\"rename\"}");
+    ewmh_update_desktop_names();
+    ewmh_update_desktop_viewport();
+    ewmh_update_current_desktop();
 }
 
 /*