Fix re-opening GRF/script settings windows not closing drop down windows

Causing assertion failure on drop down item selection

See: #527

src/script/script_gui.cpp

@@ -325,6 +325,11 @@ struct ScriptSettingsWindow : public Window {
 		this->RebuildVisibleSettings();
 	}
 
+	~ScriptSettingsWindow()
+	{
+		HideDropDownMenu(this);
+	}
+
 	/**
 	 * Rebuilds the list of visible settings. AI settings with the flag
 	 * AICONFIG_AI_DEVELOPER set will only be visible if the game setting