Improve SDK documentation and exporter checks for Java bosses

[thomasschuerger]

During development of a new multiplayer game for CG, I've learned the hard way that

• the boss file config/Boss.java must contain the class Player instead the class Boss (why?)
• the boss file config/Boss.java will not work if you use "public class Player" instead of "class Player" (again, why?)

This is not obvious at all and this is not documented anywhere in the SDK documentation. There is also no hint in the example games that are available. The example games don't make these mistakes, but a comment next to "class Player" would have helped.

With one or both mistakes in the boss file, you can export the project without an error shown, upload it as a contribution on the CG website and have it built by CG without any problems. However, afterwards in the CG IDE, the boss always produces a timeout and no other error is shown.

Please add this information clearly to the SDK documentation. Please also add this to the checks done by "Export Project". And (maybe) also add a check to the build process itself. It is easy to make these mistakes and it is hard to fix them without guidance.