Parse all branding colors from appstream (elementary#2140)

[edwood-grant]

Short summary

Aims to fix #2140

This will parse all the branding possibilities of the metadata info according to the freedesktop docs here.

The branding tag has a must-have color tag with primary_color attribute (always set its value to primary), with an optional scheme_preference attribute which shows a preference for a particular color scheme (withe light or dark) and this should take precedence.

But, as the issue says, there can be multiple setups here, and this PR aims to account for all of these in the following order:

• If the color tag has a scheme_preference, either set to light or dark, it should be taken as precedent, according to the desktop current color scheme (via gtk_application_prefer_dark_theme).
• If not, look for the color tag that has no scheme_preference (i.e. unknown). This was the original call.
• If the last call returns null, use the local color scheme setting.

Tests

Apps that I tested with this change:

• Reco: Has all color branding setups (light, dark and without scheme_preference attribute). Different colors per tag.
• KonbuCase: Only has light and dark branding tags. Different colors per tag.
• Larawan: I believe it has no branding tag of any kind.
• Mail: Has all tags, but all with the same color.

I couldn't find an application that had only the no scheme_preference attribute, but it was the default original call, so it should cover that case as well.

[danirabbit]

Thanks for your branch! This looks really good besides one problem: it caches the brand color so if you change between light and dark mode you get the old color even if you exit and re-enter the page.

I think the way to fix this would be to cache each color separately and then only check if its null after we figure out which color we're looking for

[edwood-grant]

Ah, I see, got it!

So what I did now was to create separate color values for each primary color as private members, that is:

• Primary light
• Primary Dark
• Primary Unknown (the one without any scheme preference)
• Fallback (when everything fails, the settings appcenter primary)

This is encapsulated in a private method (cache_primary_colors), which runs before asking for available colors. This should guarantee it to work every time you switch to dark mode and then enter/exit the page.

Hopefully this works better! 👍

[danirabbit]

Works here!