VENOM-443: Prepare for persistent groupchats

[naxuroqa]

• Prepare for conferences saving TokTok/c-toxcore#1156 to be merged
• Restore known groupchats from chatlist

[codecov-io]

Codecov Report

Merging #443 into release-0.5.1 will decrease coverage by 0.01%.
The diff coverage is 0%.

@@               Coverage Diff                @@
##           release-0.5.1    #443      +/-   ##
================================================
- Coverage           5.92%   5.91%   -0.02%     
================================================
  Files                 78      78              
  Lines               5249    5259      +10     
================================================
  Hits                 311     311              
- Misses              4938    4948      +10

Impacted Files	Coverage Δ
src/tox/ToxSession.vala	0.54% <0%> (-0.01%)	⬇️
src/tox/ToxAdapterConferenceListener.vala	0% <0%> (ø)	⬆️

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