Merge #16254: qt: Set AA_EnableHighDpiScaling attribute early

099e4b9ad3 Set AA_EnableHighDpiScaling attribute early (Hennadii Stepanov)

Pull request description:

  Running `bitcoin-qt` compiled against Qt 5.12.4 causes a warning:
  ```
  hebasto@bionic-qt:~/bitcoin$ src/qt/bitcoin-qt
  Attribute Qt::AA_EnableHighDpiScaling must be set before QCoreApplication is created.
  ```

  This PR fixes this issue.

  From Qt docs:
  - [Qt::AA_EnableHighDpiScaling](https://doc.qt.io/qt-5/qt.html#ApplicationAttribute-enum):
  > Enables high-DPI scaling in Qt on supported platforms (see also High DPI Displays). _Supported platforms are X11, Windows and Android._ Enabling makes Qt scale the main (device independent) coordinate system according to display scale factors provided by the operating system. This corresponds to setting the `QT_AUTO_SCREEN​_SCALE_FACTOR` environment variable to 1. This attribute must be set before `QGuiApplication` is constructed. This value was added in Qt 5.6.

  - [QCoreApplication::setAttribute()](https://doc.qt.io/qt-5/qcoreapplication.html#setAttribute)

ACKs for commit 099e4b:
  MarcoFalke:
    ACK 099e4b9ad3d9967051d5c3d45c6315d1b30fea05
  jonasschnelli:
    utACK 099e4b9ad3d9967051d5c3d45c6315d1b30fea05
  fanquake:
    ACK 099e4b9ad3d9967051d5c3d45c6315d1b30fea05. Did some testing on `Bionic` and `Windows 10` (using VirtualBox). I couldn't see any obvious visual difference, but given Marco's screens above, this change is obviously better. I also checked that there wasn't any sort of regression on macOS.

Tree-SHA512: 1965a427ee14ffb3871bac317685032406cf02d1fa2b2dc11c8b643bfe4ba09195674d149d1e41752f14c0d000446b35e142f3ce60d987ba97082fd7ee39a094

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