Create stage mod.

[andy840119]

Because one beatmap might be able to support multiple stage.
So will need to create mod to force use target mode in the gameplay.

What should be done:

1. create base abstract class mod like KaraokeStageMod.
   • should implement supported stage type.
   • should implement auto-create stage. Maybe use another flag or throw the exception if the mod is not support to create the stage?
2. Implement the ClassicStageMod.

And we expect that:

1. if there's no stage in the beatmap, should do something in the gameplay:
   • create default stage
   • throw exception(not very sure because it might be affect to the editor also.).
   • show some warning in the gameplay, like: There's no stage in the gameplay, please select the mod to force create one.
2. if we select the mod(e.g. ClassicStageMod) and entry to the gameplay:
   • should use the classic stage if contains this stage in the beatmap.
   • should auto create the classic stage into the beatmap if not have this stage in the beatmap.

[andy840119]

Another benefit for the stage mod:

• Can have the config for the target stage.

[andy840119]

Implemented in the #1960.