Track fixes for webkit2gtk

Two were really only adressed in 2.22.6 upstream, other are in earlier
uploaded versions. Track accordingly when the fix entered unstable for:

>      + CVE-2019-6226 (fixed in 2.22.0)
>      + CVE-2019-6233 and CVE-2019-6234 (fixed in 2.22.4)
>      + CVE-2019-6216, CVE-2019-6217, CVE-2019-6227 and CVE-2019-6229
>        (fixed in 2.22.5).
>      + CVE-2019-6212 and CVE-2019-6215 (fixed in 2.22.6).