Beatmap file format and IO

[HelloYeew]

Create a basic implementation of beatmap file and basic CRUD with the game's storage.

[HelloYeew]

Now the game already support basic read beatmap operation from <game folder>/beatmaps folder, the main beatmap information file is .rks file and the planned beatmap file is .rbm file. Inside the file have JSON structure deserialized from BeatmapSet class.

[HelloYeew]

From PR #17, the beatmap can be read from the beatmapset folder (.rkb file), still using the serialize Beatmap class like how BeatmapSet done. But this need to be seperate since now the Beatmap use on a lot of place from the JSON serializer to the game's database that currently when reading the beatmapfile on open the game the note has been readed and cached on the memory, that's the next step to decrease the usage of Beatmap and BeatmapSet class to a proper place.