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>

CHANGELOG.md

@@ -44,6 +44,7 @@
 - Dev: Moved preprocessor Git and date definitions to executables only. (#4681)
 - Dev: Refactored tests to be able to use `ctest` and run in debug builds. (#4700)
 - Dev: Added the ability to use an alternate linker using the `-DUSE_ALTERNATE_LINKER=...` CMake parameter. (#4711)
+- Dev: Removed `getApp` and `getSettings` calls from message rendering. (#4535)
 
 ## 2.4.4