uploader: Upload Window can be set to modal in Plugin config

Dear Harald,

Just updated the Uploader Plugin Window code so that one can configure the Upload progress window modal:

```
{
xtype: 'uploadbutton',
                    plugins: [{
                        ptype: 'ux.upload.window',
                        title: 'Upload',
                        width: 520,
                        pluginId: 'uploadWin',
                        modal: true // <<------------
                    }]
}
```