Qt 6: High-DPI support

Medium/large tasks in scope for Qt 6 (6.0).

• Enable high-DPI support by default: QTBUG-83092
• Improve auto-test coverage: QTBUG-83094
• X11: Figure out DPI configuration. Try to get away from configuration via environment variables, except for end-user overrides: QTBUG-84082
• QIcon overhaul: QTBUG-85885
• Iterate on/refactor the QHighDPIScaling API
• Documentation: needs significant changes, if not a complete rewrite. QTBUG-84250 and QTBUG-84249.
  • Rework main high-DPI page.
  • Qt documentation often use the term "pixels", which now can mean either device independent or device pixels, also depending on whether Qt::AA_EnableHighDpiScaling is enabled. Figure out how to best document this.

Deferred (6.x):

• Screen, DPI and DPR change events: QTBUG-83680
• Embedded EGLFS: verify that DPI configuration works as expected
  • If the device is using Wayland or X11, then configure that windowing system
  • Backends like LinuxFB can be configured with physical size, which works well since the target device is known.
  • Details at: https://doc.qt.io/qt-5/embedded-linux.html
• QHighDpiScaling code structure: QTBUG-83917
• Finish the high-DPI QUIP.