Implement CurrentStageInfo in the karaoke beatmap.

[andy840119]

We need a place to store which stage we are currently using in the gamplay/editor.

Guess the bast solution is Add the CurrentStageInfo into KaraokeBeatmap:

• beatmap processor will get the value from here.
• Should copy default beatmap stage info into here as default.
• Should assign the generated beatmap stage info into this property if the stage info is created by the mod.

What if the case if CurrentStageInfo and Stages in the beatmap are empty in the beatmap processor:

1. might throw the exception for asking user to select the mod in the gameplay.
2. hint the user that there's no stages in the editor.

Or maybe we should create something like DefaultStageInfo or FallbackStageInfo as default? Not very sure. -> see the idea in the #1906

[andy840119]

Implemented in the #1922.