Enable automatic high-DPI scaling for the Qt installer

Qt's high-DPI support has been getting better and better version by version. Automatic high-DPI scaling has been recently enabled (for all platforms) in the most prominent Qt tools. I'd suggest doing the same for the Qt installer to make it look a lot better on high-DPI screens and that way to give new users a better first impression of Qt.

Automatic high-DPI scaling can be enabled by calling:

QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

somewhere in the beginning of main().

Qt Creator:

• https://codereview.qt-project.org/#/c/194585/

Assistant, Designer & Linguist:

• https://codereview.qt-project.org/#/c/194970/
• https://codereview.qt-project.org/#/c/195381/