feature: GDM light/dark wallpaper

[fiftydinar]

I'm aware that this is not possible without using some hacky solutions, like using glib-compile-resources whenever light/dark mode is changed, but It would be useful to have this.

This is an upstream issue, but I don't know how you handle those, as some projects leave them open with upstream tag or close them.

I usually prefer to let those issues live with upstream tag, as they are still related to the project.

[fiftydinar]

This one should be possible to implement now.

This extension does modifications to GDM in real-time, as compiling resources is not mandatory since Gnome 45.
https://github.com/PRATAP-KUMAR/gdm-extension