ui/ffi: add timestamp to reaction group senders

[aringenbach]

Expose the timestamp for each sender on a reaction group. Fixes #1792

[codecov]

Codecov Report

Patch coverage: 86.76% and project coverage change: +0.06 🎉

Comparison is base (4e52125) 76.78% compared to head (75cd565) 76.85%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2153      +/-   ##
==========================================
+ Coverage   76.78%   76.85%   +0.06%     
==========================================
  Files         169      169              
  Lines       17822    17852      +30     
==========================================
+ Hits        13685    13720      +35     
+ Misses       4137     4132       -5     

Impacted Files	Coverage Δ
...rates/matrix-sdk-ui/src/timeline/event_item/mod.rs	64.42% <ø> (ø)
crates/matrix-sdk-ui/src/timeline/event_handler.rs	76.71% <79.31%> (+2.51%)	⬆️
crates/matrix-sdk-ui/src/timeline/inner.rs	73.33% <90.90%> (+0.16%)	⬆️
...s/matrix-sdk-ui/src/timeline/event_item/content.rs	62.25% <100.00%> (+0.37%)	⬆️

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[Hywan]

The patch doesn't explain why we need to add a timestamp to reaction. Apart from that, the code looks generally good. Tests for timestamps are missing: yes timestamps are created, but they aren't tested on their own. Can you please add these :-)?
I would also rebase the commit history a little bit if possible.

[Hywan]

Looks good. Thanks.

[jplatte]

Thanks! Ready for merge, but let's wait a bit (until our current release blocker PR is merged).

[aringenbach]

@jplatte Before merging this, there is an issue around timestamps for reactions.
When loading a room, reactions often have a timestamp that is equal to now() instead of the actual value. This seems like it is a synapse / sliding sync issue, but perhaps it makes sense to wait on this change integration, until we figure it out and fix it ?

[aringenbach]

I think this should be ready to go, the issue came from this code actually, it was because when we create the initial timeline items we put aside reactions that are step upon before the message they relate to and resolve these later. These were not stored with their timestamp in my initial implementation.

Fix commit is 1cee900
I also added a test covering that issue just in case: 6c835d0