refactor: add Channel::addSystemMessage function (#5500)

src/common/Channel.cpp

@@ -120,6 +120,12 @@ void Channel::addMessage(MessagePtr message,
     this->messageAppended.invoke(message, overridingFlags);
 }
 
+void Channel::addSystemMessage(const QString &contents)
+{
+    auto msg = makeSystemMessage(contents);
+    this->addMessage(msg);
+}
+
 void Channel::addOrReplaceTimeout(MessagePtr message)
 {
     addOrReplaceChannelTimeout(

src/common/Channel.hpp

@@ -83,6 +83,8 @@ public:
                     std::optional<MessageFlags> overridingFlags = std::nullopt);
     void addMessagesAtStart(const std::vector<MessagePtr> &messages_);
 
+    void addSystemMessage(const QString &contents);
+
     /// Inserts the given messages in order by Message::serverReceivedTime.
     void fillInMissingMessages(const std::vector<MessagePtr> &messages);