Allow typing to select items in various panels (sort, user, signal).

Factored code from the SignalsPanel to apply to all selections from main screen.
Closes feature request #3425304.

Panel.h

@@ -49,6 +49,7 @@ struct Panel_ {
    bool needsRedraw;
    RichString header;
    Panel_EventHandler eventHandler;
+   char* eventHandlerBuffer;
 };