Improve folder app icons, try to locate them from desktop file

This leverages KService to locate a rough match for an actual application desktop file, and load the app icon from that if specified. Not every application has an icon named like their key, NeoChat is just "neochat" whilst the app icon is named "org.kde.neochat". Now, the app icon can load instead of falling back to a folder icon:

The old path is still there, on my machine Nextcloud doesn't specify an icon properly so having it fallback to QIcon::fromTheme("Nextcloud") is still wanted.

This is still WIP, I don't really like the idea of listing every KService just to fetch app icons - there isn't any visible performance impact but it just feels wrong to me.