player: give FrameBuffer class its own file

The Frame buffer class is in the same js file as the PlayerWorker class. Until now they have only been use together but now FrameBuffer needs its own file because it is going to be used with the Fetcher class.

At the same time, the codecs enum should be given its own file so that it can be imported by other modules.

MJPEGLexer should also be given an import statement so that it can be imported by other modules.