Fix ##1067: Add a simple CrossProject mechanism

Yes. But if the user changes the `crossType`, I wont find the setting again --> I have a problem. That's the same reason why the final projects are mutable.

Not being able to tweak the `sharedSrcSettings` is one of the inherent limitations of this thing. Just like not being able to choose the JVM/JS project IDs...

As I'm thinking of this, maybe it is better to not have anything in `CrossProject` at all (except for `_jvm` and `_js` of course...)

I'll be in the lab in 30. Let's talk then.