Javadoc improvements, etc

[tlf30]

It would be great to have stuff documented in javadocs. What did you have in mind?

Also if you want to help with other things, there is a list of issues that we would like help with:

The Contribution Mega-Thread

Adding jme3-lwjgl3 library not included in the SDK · Issue #193 · jMonkeyEngine/sdk · GitHub there. The library has to be added into the gradle dependencies. When you look into the gradle file you see how it’s handled. The only difficulty is ensuring to not make the sdk run on lwjgl3 (or both, I don’t know what it would pick then), so it has to be something optional I think