MuseScore 2.0 branding

If we want to change the branding of MuseScore for 2.0, we need

1. A new splash screen. See #21035: [2.0] Splash screen
2. New application icons for mac, icns, don't forget 1024x1024
3. New document icons for mac, icns, don't forget 1024x1024. See Mac OSX guidelines
4. (New DMG background)
5. New ico file for windows with application and document icons. See mscore.ico and mscorefile.ico.
6. NSIS banner for the exe install package
7. Wix installer banner and background
8. data/mscore.png, 64x64, at least used in the title bar on windows
9. Full length logo for About page.

For Linux, I'm not sure what is used. To be completely safe, we should generate squared PNGs for the logo with the following dimensions.

• 16x16
• 20x20
• 22x22
• 24x24
• 32x32
• 36x36
• 40x40
• 48x48
• 64x64
• 72x72
• 80x80
• 96x96
• 128x128
• 192x192
• 256x256
• 512x512
• 1024x1024