Improve tracing of changes in DST files - format DST json in pretty print instead of single line in order to see changes more easily in DIFF about egon.io HOT 3 OPEN

Sry it won't be that straightforward because of:

1. Escaping is used without really requiring it everywhere. E.g. attribute names are escaped, rather then attribute values
2. Mix of JSON and XML into same file

But we could at least pretty print the "dst" part of the domain model. That is actually the essential part.

I don't quite understand why it requires to escape the JSON in that part of the file like it. If we want to support freeform text and are concerned about the formatting of that text we can just escape that part. Not the entire dst section as a single string.

from egon.io.

With the angular port and subsequent renaming of the project to Egon, the default-File type will be changed to .egn and .egn.svg respectively.
The new Filte-Type .egn will be human readable / properly formatted, but won't be compatable with versions previous 1.5.
For backwards-compatability, we wil provide a .dst download and update the old Verion to 1.5 to be able to read .egn Files

from egon.io.