Add clickable fold icon to gutter

This adds a clickable fold icon in the gutter of the react editor; refs #1984 and #2423.

This also simplifies the `GutterComponent`. I did not notice performance differences after following the benchmark instructions in https://github.com/atom/atom/pull/2325#issuecomment-43952215 (but I may have misread).

If the `GutterComponent` changes are not acceptable I can resubmit without them.

Screenshot:

![image](https://cloud.githubusercontent.com/assets/253202/3183215/08a7f0fa-ec65-11e3-91de-209509dc4a6e.png)