Migrate opsu! to lwjgl3 / libGDX

[CloneWith]

Have done some research before... and I need quite some time to learn.

I'll keep maintaining this repo, and will experiment some new features here.

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Listing here for convenience:

• Dependency update and cleanup
  • CloneWith/opsu-next#9
  • Find an alternative of Slick2d relevant elements
• Multi-platform support
• Framework Initialization
• Input handling
• Files / configuration I/O
• Assets management
  • Images
  • Music and sounds
  • Fonts
• Animations and transitions

[CloneWith]

For now I'll keep on this version of opsu!, since it is really hard to pick the proper dependencies and so much to consider!