ImGuiStructDestroyable should implement AutoCloseable

[Oondanomala]

I'm not sure if there's a reason why this isn't already the case, but if there isn’t then it would be nice to implement a close method that calls destroy so we can use it in try-with-resources statements.

[Oondanomala]

Also some of the callback classes in imgui.callback should be interfaces so we can use lambdas (again, assuming there isn't a reason why they're not interfaces, I don't really understand JNI)