v4.0 re-design

[MikuAuahDark]

There's plan to completely redesign the user interface in v3.1 to be more material-based. Currently we have one of Lovewing team giving suggestion to the design, and I'll slowly implementing it.

Progress can be seen in over_the_rainbow branch.

What's currently re-designed:

• Splash screen

• Loading screen

• Main menu

• Change units interface

• Beatmap selection interface

• Result screen interface

• Another new Live Show! UI

• Settings

• Download beatmaps interface