Separation

Okay, I know this is a pain of PR...

At the end of these 170 commits, OpenImporter is finally (hopefully) able to convert from any two systems (provided there is a converter lol).
There is still quite a bit to do ((automated) testing anyone? :stuck_out_tongue: ) but at least the skeleton is there.

cc @Nao and @pandos I added also the converters to Wedge I found in the importer repository, unfortunately I didn't have time to test them and I don't know if you are interested in join the efforts or if you prefer to go with the custom version.

Later on I'll try to convert and set up some of the converters to SMF as well (it shouldn't be difficult).
cc @oldiesmann @dragooon @live627 @margarett since you worked on SMF converters, you may find (or not) interesting the idea.