Enable high-DPI support by default on on all platforms. This removes the need for boilerplate setAttribute() calls in application and example code. Applications that wants to use different defaults can still include the calls with the desired settings (using API already in Qt 5.15).

Suggested changes:

• Enable AA_UseHighDpiPixmaps: (DONE) https://codereview.qt-project.org/c/qt/qtbase/+/273954/1
• Enable Qt::AA_EnableHighDpiScaling: https://codereview.qt-project.org/c/qt/qtbase/+/273952/1
• Don't round scale factors. (e.g. Windows at 150% -> scale 1.5x) https://codereview.qt-project.org/c/qt/qtbase/+/273956/1