[WIP] Private server menu

I added a new feature to let users edit their own server list, called Private Server List, as opposed to the "Public" ones provided by Xonotic master.

The original customizeable address bar has some bug such as https://gitlab.com/xonotic/xonotic-data.pk3dir/issues/1463 as well as some problems such as: . the servers don't appear if they don't answer to direct query . the servers cannot be seen by the users when they are offline . there is no way to edit those servers.

This feature allows users to add private servers by their addresses, and they can be assigned a name for easier identification.

Right now I'm asking for feedback, review, and testing from the devs and the community, in the Xonotic forum: http://forums.xonotic.org/showthread.php?tid=5563