Fix crash that could occur when closing a split before its display name was updated (#4731)

This fixes a crash that could occur when closing a split before the
display name had a chance to update

The reason I found this was because the LiveController change I made
updated display names more regularly

Additionally, we now make sure to not send duplicate `displayNameUpdated` signals
upon each request for channels with CJK characters in their display name

* Default-initialize the `actualDisplayName` with the user's login name to not send an initial display name update if the display name is the same casing as the login name

src/widgets/splits/Split.hpp

@@ -163,6 +163,10 @@ private:
     pajlada::Signals::Connection roomModeChangedConnection_;
 
     pajlada::Signals::Connection indirectChannelChangedConnection_;
+
+    // This signal-holder is cleared whenever this split changes the underlying channel
+    pajlada::Signals::SignalHolder channelSignalHolder_;
+
     pajlada::Signals::SignalHolder signalHolder_;
     std::vector<boost::signals2::scoped_connection> bSignals_;