* refactor: remove singletons from message rendering
* chore: add changelog entry
* Disable the `cppcoreguidelines-avoid-const-or-ref-data-members` clang-tidy check
* auto *app
* Selection is a struct, not a class
* Use ChannelView's `signalHolder_` instead of `channelConnections_`
* Remove `applySettings` step, instead just connect & set each setting individually
* rename & constify some context values
* Handle empty "last message color" setting value better (as it was
originally in this pr before I removed that change :-)
* unrelated mini refactor cleanup
* let painSelection handle size_t instead of int
* Add some more comments to the MessageLayoutContext structs
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Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>
