Fixed scrolling when QuickLook is opened in Elements panel

[romansp]

So basically this fixes #18. If you can review this, this will be great.

ElementTreeItemWrapper was required, because ElementTreeItem is rendered recursively.

This is 100% going to be rebased I think, as PR also includes change to webpack.skpm.config.js that was required to get dev tools running on older mac, but most probably you want this to be reverted.

[mathieudutour]

haven't reviewed yet but there were some discussion about transpiling cocoascript-class, I don't think it's as simple as running babel on it: airbnb/react-sketchapp#106 (comment)

[romansp]

Yeah, saw that, but at least I can run it instead of erroring during start.

In my own plugin with webview I had to use a safer version https://github.com/atlassian/cocoascript-class for proper transpiling.

It's totally fine to exclude webpack.skpm.config.js change from final PR. I will rebase as soon as you are satisfied with actual PR.