Fix XML parsing so that all valid encodings are supported

Currently only ASCII encoding was supported even if `<?xml version="1.0" encoding="utf-8"?>` was specified, because encoding was taken as system's default ignoring this.

This PR fixes it so all encodings are supported and this header is correctly parsed.


[I created my theme](http://eclipsecolorthemes.org/?view=theme&id=19553) but because author is `Dāvis` it didn't imported it with error `This is not a valid theme file.`

Theme name can also contain any Unicode characters and works good in Eclipse