GlueGen JavaCallback: Resolve key mapping of callback and associated resources via 'JavaCallbackKey' config and custom `SetCallback-KeyClass`
   
    Updated unit test and doc accordingly.
    Unit tests handle OpenAL's AL_SOFT_callback_buffer and AL_SOFT_events.
   
    Tested global scope (no key, default) and 1 key (default) and 1 key (custom class).
   
    Added more query functions, which all only take the `SetCallbackFunction` key arguments as specified.
   
    Cleaned up JavaCallback* config class field naminig scheme.
   
    Added 'synchronized (..Map) { }' block in crucial `SetCallbackFunction`,
    rendering implementation thread safe.