Skip unprocessed keydown events in the styling assistant so that typing style names actually works

aegisub/src/dialog_styling_assistant.cpp

@@ -259,4 +259,6 @@ void DialogStyling::OnKeyDown(wxKeyEvent &evt) {
 		if (from > 0)
 			style_name->SetSelection(from - 1, to);
 	}
+	else
+		evt.Skip();
 }