Read properties files as UTF-8

[jowerner]

At the moment, XLT reads properties files using the ISO-8859-1 encoding. This is for historical reasons only and the only case where UTF-8 is not used yet. Time to go with UTF-8 for properties files as well. This should also simplify maintaining properties files with non-Western text content. No need to deal with unicode escapes any longer!

Note that this is another breaking change that needs to be documented in the migration guide for XLT 7.