Use <map> node version attribute for feature-signalling/format version

[Yanrishatum]

Since Tiled developing new features, it's becoming more and more important to store format version in the TMX file. So far XML do not contain any explicit attribute/node to tell which format used and which features are available.
I propose using version attribute as explicit format version for XML, removing any guessing on format version on parser side. This attribute is here for very long time and always had value of 1.0, hinting that it was planned to use it as format version, yet it never used that way.

[bjorn]

I should have been doing this since a long time, but I planned to start incrementing this version attribute once reaching Tiled 1.0. Fortunately, that version is not far off anymore (should be early next year).