Use SDL_WINDOWEVENT_DISPLAY_CHANGED to notice about window switching display to update _density an dpi

[misl6]

Maintainer merge checklist

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.
• Unittests are included in PR.
• Properly documented, including versionadded, versionchanged as needed.

🔴 It was supposed to be "Ready to Merge", but the CI is failing. We should consider how the SDL2 dep is managed on Ubuntu and other Linux distros.

⚠️ SDL_WINDOWEVENT_DISPLAY_CHANGED has been added on SDL 2.0.18 but needs SDL 2.24.0 to work as expected on macOS, as previously was using a non-macOS compatible logic to compute the display on which the window is really attached to (#7974 bumps SDL2 to 2.24.0).

💡 Since now SDL2 seems to be aware of DPI on all platforms (including Windows), we may want to update how dpi gets computed.

• Added missing SDL_WINDOWEVENT_* declarations for future usage.
• Handle SDL_WINDOWEVENT_DISPLAY_CHANGED so we can notice about a window that changed the display is attached to.
• Extract a method _update_density_and_dpi which updates _density and dpi to avoid DRY violations

Previously:

After:

[FilipeMarch]

Hello, since 2022 I would like to use my external monitor, but because of this dpi bug I can't because the screen gets completely distorted here using macOS. Please could you merge this PR? This would really really help me a lot.