generator: Create a directory for each namespace

When dealing with several namespaces there might be classes with the same name
(especially Global which is autogenerated in g-i based bindings). On generation
the file would be overriden by the last occurence in the xml. To encounter
this every namespace has it's own directory now. This also improves structure
a lot when dealing with big libraries.

Also do the necessary adaption for the build and the csproj files.