Support to install bundled assets when application's sources are extra-data

[x80486]

Checklist

• [X] I agree to follow the Code of Conduct that this project adheres to.
• [X] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Suggestion

Currently, there is no real path to include and install bundled assets when application's sources are extra-data — for instance: flathub/com.syntevo.SmartGit/pull/113

This is somehow counterintuitive to the packaging process because reusing application's assets should be the first option, rather than maintaining copies.

I suggest implementing a mechanism that allows developers to specify bundled assets in Flatpak manifest when application's sources are extra-data, which would be then installed alongside the application.

[bbhtt]

Flatpak builder supports it, see the note in https://docs.flatpak.org/en/latest/module-sources.html#extra-data

It's not allowed on Flathub specifically.

[x80486]

OK, then I guess I should move this one out to the flathub repository :sweat_smile:

[bbhtt]

We don't have any plans to support it, it was blocked specifically so that we can validate all the metadata.

[bbhtt]

And also the default appstreamcli compose command does not work when you place files ie. icon desktop and metainfo there