Advanced channel filters (#1748)

Adds custom channel filters complete with their own mini-language. Filters can be created in settings, and applied by clicking the three dots to open the Split menu and selecting "Set filters".

src/common/Channel.hpp

@@ -72,6 +72,7 @@ public:
     void addOrReplaceTimeout(MessagePtr message);
     void disableAllMessages();
     void replaceMessage(MessagePtr message, MessagePtr replacement);
+    void replaceMessage(size_t index, MessagePtr replacement);
     void deleteMessage(QString messageID);
     void clearMessages();