fix(iTip): Prevent single-occurrence reply to be a significantChange for everyone

[DerDreschner]

Under some circumstances (e.g., replying to a single event in a recurring series with a lot of participants), all participants get a new REQUEST message delivered. It's pretty confusing for them, as this shouldn't happen. This is being fixed with this PR.

[DerDreschner]

CC @SebastianKrupinski @provokateurin

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.61%. Comparing base (fb5863d) to head (cd9878c).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ITip/Broker.php	94.11%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #784      +/-   ##
============================================
- Coverage     98.66%   98.61%   -0.05%     
- Complexity     1759     1783      +24     
============================================
  Files            71       71              
  Lines          5165     5213      +48     
============================================
+ Hits           5096     5141      +45     
- Misses           69       72       +3     

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