Handle different sequences in base event and exception event

[gharlan]

The sequence in exception events can be different to the sequence of the base event. But the ITip broker always replies with the base sequence number, even for replies to exception events.

The pull requests shows the bug.

From iCalender RFC:

Recurrence instances of a recurring component MAY have different sequence numbers.

https://datatracker.ietf.org/doc/html/rfc5545#section-3.8.7.4

Google Calendar uses different sequence numbers for exception events.
Example: Base event has sequence 2. When creating a new exception event, it gets the sequence number 3, but the base event keeps number 2.
Now when responding to the exception event via VObject iTip broker, the response contains the wrong sequence number 2, so the response is ignored by Google calender.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.77%. Comparing base (2ce84f7) to head (b4c8205).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #715   +/-   ##
=========================================
  Coverage     98.77%   98.77%           
- Complexity     1884     1885    +1     
=========================================
  Files            71       71           
  Lines          5289     5291    +2     
=========================================
+ Hits           5224     5226    +2     
  Misses           65       65           

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[gharlan]

Added a fix now.