Introduce MsgLike on the FFI layer

[stefanceriu]

This patch introduces an intermediary MsgLike Content and Type on the FFI layer and brings the timeline items in line with the ones from the UI crate.

This is the last step in the timeline item structure factoring, following #4839, #4853 and #4860

Best reviewed commit by commit.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.69%. Comparing base (3ece8e6) to head (c29b833).
Report is 19 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4902   +/-   ##
=======================================
  Coverage   85.69%   85.69%           
=======================================
  Files         309      309           
  Lines       35398    35398           
=======================================
+ Hits        30333    30336    +3     
+ Misses       5065     5062    -3     

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[Hywan]

Noiiiiice!