Multi-display / multi-monitor support in ReactOS

Since ReactOS is slowly getting multi-monitor support, it makes sense to have this ticket for tracking the progress.

Work is mostly being done by Hervé Poussineau: currently he is working on supporting multiple graphic adapters (testing with QEMU's QXL), but only one monitor per adapter.

• Multi-monitors in Display Properties: message
• Preparatory work for multi-display: message, PR #4408 (merged)
• Screen panning: message, PR #4509 (merged)

Some documentation is available on wiki:

• https://reactos.org/wiki/Google_Summer_of_Code_2020_Ideas#Multi-Monitor_Support
• https://reactos.org/wiki/Techwiki:Win32k/display_driver_loading
• https://reactos.org/wiki/Techwiki:Win32k/GRAPHICS_DEVICE
• https://reactos.org/wiki/Multidisplay_support_in_ReactOS (stub)