Remove getApp and getSettings calls from Message-Rendering (#4535)

* 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

---------

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

.clang-tidy

@@ -18,6 +18,7 @@ Checks: "-*,
   -cppcoreguidelines-pro-bounds-array-to-pointer-decay,
   -cppcoreguidelines-owning-memory,
   -cppcoreguidelines-avoid-magic-numbers,
+  -cppcoreguidelines-avoid-const-or-ref-data-members,
   -readability-magic-numbers,
   -performance-noexcept-move-constructor,
   -misc-non-private-member-variables-in-classes,