Things depend on the react editor wrapper having the .editor class,
but inside the editor, we can control the style. I changed the
component's div to be .editor-contents for now. We can eliminate this
extra style when we eliminate the wrapper.
The space-pen view is now a simple wrapper around the entire React
component to integrate it cleanly into our existing system. React
components can't adopt existing DOM nodes, otherwise I would just have
the react component take over the entire view instead of wrapping.
We don't actually need structural markup to ensure that all pane views
are absolutely positioned. We can just use the `> *` selector inside of
.pane-items.
/cc @probablycorey is there anything I'm missing here?
The .item-views div needs to be the first child of pane and contain
the absolutely positioned repaint hack div inside it, otherwise the tabs
don't get honored as flexbox items.
