Hide Options icon when working set is hidden

This is for #9137.

Also added drop shadow to splitview icon.

If we applied classes to `#editor-holder` such as `split-view` and `horizontal-split`/`vertical-split`, then we could avoid this JS logic here (and also in WorkingSetView.js) and do this all in CSS.

We could also avoid using `#second-pane` as a selector [here](https://github.com/adobe/brackets/pull/9127#discussion_r17626809).