Proper touchscreen support

[calintat]

We really want homotopy.io to work well on touchscreens, particularly tablets.

Here are some things off the top of my head that we could do to improve touchscreen support:

• [ ] larger touch targets,
• [ ] find a way to highlight slices or attachments,
• [ ] swipe to left to edit/delete generators,
• [ ] orbit/zoom 3d/4d diagrams with touch,
• [ ] something to change the current slice,