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

doc/assets-attribution.md

@@ -49,8 +49,7 @@ Jonas Schnelli
     src/qt/res/icons/tx_output.png, src/qt/res/icons/bitcoin.icns,
     src/qt/res/src/bitcoin.svg, src/qt/res/src/bitcoin.ico,
     src/qt/res/src/bitcoin.png, src/qt/res/src/bitcoin_testnet.png,
-    docs/bitcoin_logo_doxygen.png, src/qt/res/images/splash.png,
-    src/qt/res/images/splash_testnet.png, src/qt/res/src/tx*.svg,
+    docs/bitcoin_logo_doxygen.png, src/qt/res/src/tx*.svg,
     src/qt/res/src/connect*.svg, src/qt/res/src/clock*.svg,
     src/qt/res/src/mine.svg, src/qt/res/src/qt.svg,
     src/qt/res/src/verify.svg,