[XML] Add an XML version tag

Concise description:
It seems that a recurring problem with suggestions for changes to the XML or game rules is determining if it is going to break any existing maps. If a tag was added to the start of each map XML file specifying which version it was written for, then the game code for that version could be locked, and new features would be guaranteed to have no effect on anything that came before it.
Specifics/Details:
It seems that a recurring problem with suggestions for changes to the XML or game rules is determining if it is going to break any existing maps. If a tag was added to the start of each map XML file specifying which version it was written for, then the game code for that version could be locked, and new features would be guaranteed to have no effect on anything that came before it.
Specifics/Details:
- Add a tag <version>##.#</version> that appears right after <map>. If it is not there, assume version 1.0.
- XML and game engine changes would be easier to implement.
- Tags could be updated or retired as needed without concern for maintaining backwards compatibility.