Draft: Improve appearance of titlebars in Windows 10 and 11 with server side decorations

Adds GDK_WIN32_IMMERSIVE_TITLEBAR and GDK_WIN32_TRANSPARENT_TITLEBAR environment variables.

On Windows 10 the titlebar will also now be set to dark mode if gtk-application-prefer-dark-theme is enabled.

This mainly adds two features: "immersive titlebars" and "transparent titlebars", which are enabled via environment variables (they could obviously be renamed still if needed).

Immersive titlebars sets the titlebar colors to the GTK theme's colors on Windows 11. It looks nice with Libadwaita which looked particularly bad with the light titlebar in dark mode.

Transparent titlebars enables blur on the titlebar on Windows 11, as seen in many Windows 11 apps.