Display screenshots based on prefers_color_scheme

[meisenzahl]

I had thought about whether it would be useful to be able to annotate the screenshots in AppData as to whether they are for prefers_color_scheme dark.

<screenshot type="default" prefers_color_scheme="dark">
    <image>https://raw.githubusercontent.com/manexim/typewriter/master/data/screenshots/000-dark.png</image>
</screenshot>

Then AppCenter could display the appropriate screenshots based on the current setting:

no-preference

dark

[danirabbit]

Yeah I think we'd have to propose this upstream to the appstream spec, but I think this is a good idea 👍