khal accepts events missing the TZOFFSETTO and TZOFFSETFROM properties

Ave,

Thank you for your awesome work on khal!

It appears that khal does not sufficiently validate ICS files it is asked to import: in particular it does not verify for the presence of the tzoffsetto and tzoffsetfrom properties in the "STANDARD" and "DAYLIGHT" subcomponents of the "VTIMEZONE" component. This allows for importing events that will then break other clients after synchronization (e.g. Davx⁵).

It will also accept events without a PRODID, silently adding itself as a PRODID. I don't think that's a problem, but I just thought I'd mention it too.

Example accepted file (slimmed down from a real-world ICS offered by some random website):

BEGIN:VCALENDAR
VERSION:2.0
METHOD:PUBLISH
BEGIN:VTIMEZONE
TZID:Europe/Zurich
BEGIN:STANDARD
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
DTSTART:19810329T020000
END:STANDARD
BEGIN:DAYLIGHT
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
DTSTART:19810329T020000
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
CLASS:PUBLIC
CREATED:20200908T142600
DTEND;TZID=Europe/Zurich:20201029T221500
DTSTAMP;TZID=Europe/Zurich:20200908T142600
DTSTART;TZID=Europe/Zurich:20201029T204500
TRANSP:OPAQUE
UID:25b2ac7d-0ec9-43f2-91ba-ac869838f922
END:VEVENT
END:VCALENDAR

iCalendar validator output:

Missing required PRODID property near line # 1
Reference: RFC 5545 3.6. Calendar Components
Missing required TZOFFSETTO property near line # 6
Reference: RFC 5545 3.6.5. Time Zone Component
Missing required TZOFFSETFROM property near line # 6
Reference: RFC 5545 3.6.5. Time Zone Component
Missing required TZOFFSETTO property near line # 10
Reference: RFC 5545 3.6.5. Time Zone Component
Missing required TZOFFSETFROM property near line # 10
Reference: RFC 5545 3.6.5. Time Zone Component

OS/Distribuiton: GNU/Linux, Arch Linux
Khal version: khal, version 0.10.2
Vdirsyncer version: vdirsyncer, version 0.16.9.dev0+gb5dd092.d20200713

Please ask if I can provide any additional useful information.

Reproduce the supplied ICS example and trace khal's import and iCalendar validation entry points. Check how VTIMEZONE STANDARD and DAYLIGHT components are validated, then add coverage showing that missing TZOFFSETTO and TZOFFSETFROM are rejected; the issue is done when malformed imports no longer pass silently.