Support 'style' (light/dark) part of screenshot 'environment' property

[sophie-h]

• Maybe show dark screenshots for prefers-color-scheme: dark and light ones otherwise.
• Maybe add an option to manually switch between dark and light screenshots
• If no different styles are available, just show all available screenshots independent of prefers-color-scheme.

Hieroglyphic is an app that currently has dark and light screenshots. But they are already removed upstream. (cc @FineFindus)

apps.gnome.org/Hieroglyphic/ changes the displayed screenshots based on prefers-color-scheme.

Here is an issue for clarification on the spec ximion/appstream#667

Here is the spec

[FineFindus]

Hieroglyphic is an app that currently has dark and light screenshots. But they are already removed upstream. (cc @FineFindus)

I removed them because neither Flathub nor GNOME Software display them correctly and having duplicate light/dark screenshots looks bad and is discouraged by the Circle Guidelines. Happy to add them back as soon as this is correctly supported :)

[vmkspv]

I was about to add environment to the metainfo of my project, but now I’ll probably hold off and just remove the duplicate dark-style screenshots until this tag actually makes some sense. Really hoping that app stores will add support for it, so we can finally "join the dark side". 🌚