Use rotateplane cursor as spinner

In this PR I took the [bounce cursor](https://github.com/adobe/brackets.io/blob/gh-pages/css/spinner.css) from brackets.io and replaced the Brackets `.spinner` with a modified version of this cursor.
I think it suits better to such a modern UI, but it wouldn't be a problem to me to just use the current one furthermore.

@larz0 for UI review