Add sclass to messagebox

Purpose
======

This update enables to specify some sclass to Messagebox's window.
This allows us to change style of each Messageboxes.

as follows:

```java
Messagebox.show(
    "Do you want to return without saving?",
    "Confirmation",
    [Messagebox.Button.OK, Messagebox.Button.ABORT, Messagebox.Button.CANCEL],
    ["Save and Return", "Abort and Return", "Cancel"],
    null,
    null,
    null,
    [sclass: "myMessagebox"]
);
```