Added synchronization checks to all uses of mChildren and mPlugins

A redo of my previous pull request. This one should be isolated. For reference, the original message follows:

Extending the existing concurrency protection in RajawaliRenderer, I have added synchronization checks on all access to mPlugins and mChildren. I have tested this with all of the demos in the RajawaliExamples project. I'll admit I don't understand all of the details of the implementation of the library yes so I am not sure where multiple threads can really access the renderer but if there is reason to synchronize these in the render() method than to be thread safe I believe it makes sense to synchronize them everywhere. I also am pretty certain I did not cause multiple synchronization checks through recursive calls.