Add webview

[gin0606]

Hi @AndyQue
I try to add My WebView to cocos2d-x after lookng this issue. https://github.com/gin0606/cocos2dx-WebView/issues/2

But I have compile error from libui iOS.
I try to fix that. But I'm not sure if this is correct.
Would you please check this?

[zilongshanren]

Thanks a lot @gin0606
Sure, I will take over this PR and fix the compile errors.
BTW, in order to merge your code into cocos2d-x repository, I have to change the license and namespace part. Since the WebView only works on Android and IOS platform, so we might need to put it under cocos2d::ui::experimental namespace.

[zilongshanren]

@gin0606
The new PR is opened here: #7882 , please check it out.

[gin0606]

@AndyQue I will check it! Thank you!