IRCClient+LibGUI: Add an input box so we can send messages to channels.

Implement this using a GTextEditor with a special single-line mode.
This new mode needs some polishing, but it's already very useful.

Applications/IRCClient/IRCLogBufferModel.h

@@ -8,9 +8,9 @@ class IRCLogBufferModel final : public GTableModel {
 public:
     enum Column {
         Timestamp = 0,
-        Prefix,
         Name,
         Text,
+        __Count,
     };
 
     explicit IRCLogBufferModel(Retained<IRCLogBuffer>&&);