Move the stage info out of the karaoke beatmap.

[andy840119]

I think finally the stage will be separated.
And will use the injection (e.g. IStageProvider) to get and edit the stage.

Will use the Workingbeatmp.GetStream() to read the stage file.

[andy840119]

Need to make sure that working beatmap is able to save the file.

[andy840119]

Also, we need something like StageChangeHandler to undo/redo the stage.

[andy840119]

I think will start implement this after:

1. Implement the preview stage info.
2. Lazer is able to save/load customized beatmap.

It's too early for thinking about this.

[andy840119]

Pin this issue for not forgot it.

[andy840119]

Also, should think about:

• how to trigger the drawable hit-object to re-calculate the working stage(maybe trigger by the change handler or in the stage)? -> maybe use working beatmap to load the stage info from the json file and put into the karaoke beatmap? Not very well but it's a workable way.