Extract the tcell-to-gocui event conversion out of pollEvent

A following commit needs pollEvent to attach information from the
replayed-event wrappers to the GocuiEvent it returns. With the
conversion inlined there is no seam to do that in, because every branch
of the type switch returns directly.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pkg/gocui/tcell_driver.go

@@ -300,6 +300,10 @@ func (g *Gui) pollEvent() GocuiEvent {
 		tev = <-Screen.EventQ()
 	}
 
+	return gocuiEventFromTcellEvent(tev)
+}
+
+func gocuiEventFromTcellEvent(tev tcell.Event) GocuiEvent {
 	switch tev := tev.(type) {
 	case *tcell.EventInterrupt:
 		return GocuiEvent{Type: eventInterrupt}