Support alignment in CCUI linear layout

[ntrrgc]

This allows to use left, center or right alignment in HBox nodes and top, center or bottom alignment in VBox nodes.

[zilongshanren]

@ntrrgc

This PR looks good to me. It is really an useful feature when dealing with Layout. But I suggest to add it to the C++ engine and make sure the Javascript bindings works.

And please also add an example to demonstrate this feature in https://github.com/cocos2d/cocos2d-x/tree/v3/tests/js-tests

If anything is unclear here, I would like to help.

Thanks.

[pandamicro]

Hi, @ntrrgc is there a Cocos2d-x pr ?

[ntrrgc]

@pandamicro There you go: https://github.com/cocos2d/cocos2d-x/pull/17535

[pandamicro]

That great ! Thanks ~