Fix AppStream metadata validation

[City-busz]

This updates the AppStream metadata to a newer version, and fixes all compatibility problems detected by appstreamcli validate gourmet.appdata.xml:

I - gourmet.appdata.xml.in:gourmet.desktop:27
    Consider using a secure (HTTPS) URL for 'http://thinkle.github.io/gourmet/'

E - gourmet.appdata.xml.in:gourmet.desktop:24
    The screenshot does not contain any images.

E - gourmet.appdata.xml.in:gourmet.desktop
    The component is missing a name (<name/> tag).

E - gourmet.appdata.xml.in:gourmet.desktop:25
    The screenshot does not contain any images.

E - gourmet.appdata.xml.in:gourmet.desktop
    The component is missing a summary (<summary/> tag).

I - gourmet.appdata.xml.in:gourmet.desktop:4
    The id tag for "gourmet.desktop" still contains a 'type' property, probably from 
    an old conversion.

W - gourmet.appdata.xml.in:gourmet.desktop:28
    Found invalid tag: 'updatecontact'. Non-standard tags must be prefixed with 
    "x-".

W - gourmet.appdata.xml.in:gourmet.desktop:4
    The component ID is not a reverse domain-name. Please update the ID and that of 
    the accompanying .desktop file to follow the latest version of the Desktop-Entry 
    and AppStream specifications and avoid future issues.

And don't use legacy path for metainfo file. Metainfo files should be installed into /usr/share/metainfo. See: https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#spec-component-location