Qt: HiDPI (retina) support for splash screen

- remove splash screen images (reduce binary size)
- dynamicly draw splash screen with available icon.
- remove testnet icon
- dynamicly colorize testnet icon
Qt: Go back to using QIcon functionality for scaling
resize tray icon because a 1024x1024 icon won't show in ubuntu (bug)
Merge pull request #5228

ac23394 resize tray icon because a 1024x1024 icon won't show in ubuntu (bug) (Jonas Schnelli)
8e76ca0 Qt: Go back to using QIcon functionality for scaling (Wladimir J. van der Laan)
54f2571 Qt: HiDPI (retina) support for splash screen (Jonas Schnelli)