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>

src/widgets/helper/ChannelView.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "common/FlagsEnum.hpp"
+#include "messages/layouts/MessageLayoutContext.hpp"
 #include "messages/LimitedQueue.hpp"
 #include "messages/LimitedQueueSnapshot.hpp"
 #include "messages/Selection.hpp"
@@ -343,6 +344,9 @@ private:
 
     std::unordered_set<std::shared_ptr<MessageLayout>> messagesOnScreen_;
 
+    MessageColors messageColors_;
+    MessagePreferences messagePreferences_;
+
     static constexpr int leftPadding = 8;
     static constexpr int scrollbarPadding = 8;