desktop: Check that icon file from desktop file is actually installed

[bbhtt]

This should not be breaking as without the correct icon from the desktop file installed, appstream will not create files/share/app-info/icons/{appid}.png.

We already check that files/share/app-info/icons/{appid}.png exists and icons following the flatpak id patterns exist in files/share/icons/hicolor/*/apps/* but not if the icon defined in the main desktop file exist.

This plugs that hole.

See for example https://github.com/flathub/ml.mdwalters.Lemonade/pull/9#issuecomment-2069961956

The linter is saying:

    "errors": [
        "appstream-missing-icon-file",
        "appstream-missing-icon-key"
    ],

but the root cause is, the desktop file icon ml.mdwalters.Lemonade not existing in files/share/icons/hicolor/*/apps/*.

This is not caught by any other checks as the icon that is installed follows the exportable pattern: ml.mdwalters.Lemonade.Devel.svg

[bbhtt]

Now https://github.com/flathub/ml.mdwalters.Lemonade/pull/9#issuecomment-2069961956 should look like this, (which is better than before):

{
    "errors": [
        "desktop-file-icon-not-installed",
        "appstream-missing-icon-file"
    ],
    "warnings": [
        "appstream-screenshot-missing-caption"
    ],
    "message": "Please consult the documentation at https://docs.flathub.org/docs/for-app-authors/linter"
}