Add a setting for the timeout stack style:

"Stack" is the default behaviour, it will search 20 messages up and 5
seconds back in time to stack the timeout.
"Stack sparingly" will try to do the same, but only if the user has not
typed a message inbetween the this and the last timeout.

Fixes #1157

src/common/Channel.hpp

@@ -18,6 +18,13 @@ using MessagePtr = std::shared_ptr<const Message>;
 enum class MessageFlag : uint32_t;
 using MessageFlags = FlagsEnum<MessageFlag>;
 
+enum class TimeoutStackStyle : int {
+    StackHard = 0,
+    DontStackBeyondUserMessage = 1,
+
+    Default = StackHard,
+};
+
 class Channel : public std::enable_shared_from_this<Channel>
 {
 public: