Add and show app icons

Requested in the matrix channel today:

schmiddi: Btw, icons would probably be nice in the app listing.

Like with screenshots, adding them to git maybe disadvantageous, but, filesizes, churn and filetype (svg vs. pixel graphics) may lead to adding them to the git.

Next steps:

1. Assuming icons are referenced in appstream metadata: evaluate total file size for apps that have this metadata -> they are rarely mentioned.
2. Figure out, what to do for apps that don't make it this easy
3. Come up with design
4. Discuss/Feedback
5. Implementation

Steps (SVG) only for now - if there's many non-vector icons, adjust this:

• Add line item for this in lpa_helper: extra.svg_icon_url (see https://framagit.org/linuxphoneapps/linuxphoneapps.frama.io/-/blob/main/content/games/io.github.nokse22.trivia-quiz.md?ref_type=heads for a first occurrence)
• implement checker item to add this value when an icon is specified (see https://gitlab.com/TheOneWithTheBraid/f_wallet/-/raw/main/linux/business.braid.f_wallet.metainfo.xml for an example of Appstream Metadata that specifies an icon

Further steps, once we've got a meaningful amount of listings (100+) with an icon linked:

• come up with a local caching mechanism that downloads icons (and can be run more than once)
• integrate icons into overviews and listings, referencing the downloaded icons