Add Children directly

Instead adding elements one by one using addChild we could add a list of children directly using.

So that, instead doing:

```
var ac = new AccordionContainer({region: 'left', splitter: true});
ac.addChild(new ContentPane({title: 'left', content: 'left'}));
ac.addChild(new ContentPane({title: 'center', content: 'center'}));
ac.addChild(new ContentPane({title: 'right', content: 'right'}));
```

we could easily do:

```
            new AccordionContainer({
                region: 'left', splitter: true,
                children: [
                    new ContentPane({title: 'top', content: 'top'}),
                    new ContentPane({title: 'middle', content: 'middle'}),
                    new ContentPane({title: 'bottom', content: 'bottom'})
                ]
            })
```