Add help popup

[freeman-lab]

Many visualization types have either keystrokes or mouse actions that trigger different functionality, and much of this is currently undocumented and may not be obvious (e.g. clicking to select nodes, using arrow keys to change color map).

We should add functionality for a ? icon as a small button, maybe in the upper right of a visualization, next to the Actionsbutton.

This should be an endpoint that allows a key-value specification of buttons/keys and associated actions, which visualization types can then provide. For example, for a line we might have:

{
    "option-scroll" : "Zoom horizontally",
    "command-scroll" : "Zoom vertically"
}

For images

{
    "drag": "Draw regions",
    "option-drag" : "Modify regions"
    "command-drag" : "Pan image"
}

In some cases we'll have groups of buttons that have similar functionality, so probably want to be able to be concise, as in:

{
    "w,a,s,d" : "Pan camera",
    "left-arrow, right-arrow": "Orbit camera"
}

Would be very nice to somehow replace the key strings with icons (e.g. for left-arrow), but that can wait.

It's also not clear whether to add this to the visualization only as displayed in the lightning notebook or in a way that can be embedded, @mathisonian ?