cornerstone: add a toolbar

[kousu]

Cornerstone has a series of tools. The OHIF Viewer (the flagship app that Cornerstone was written for) exposes most of them in a toolbar:

but they can also be as simple as mouse gestures

Add something subset of tools to our renderer (#24). A static image is fine for a png, jpg, or tiff, but for NIfTIs, which are usually 3d, and sometimes 4d, it would be a lot more friendly if users could at least:

• stack scroll
• pan
• zoom

without having to download the dataset and switch to fsleyes or ITK Snap.

[kousu]

It would be great to have the multi-axis view like fsleyes and brainsprite do, like this, where navigating through the image is easy because you can just click at any point in any axis. I'm not clear if cornerstone can do this?