Add `disablemethod` flag to button and slider steps

An emerging pattern for examples of sufficient complexity is that it’s becoming necessary to hook into components via events instead of strictly the command api interface. This might be the case if you have to, say, perform a couple precisely sequenced async actions before animating.

However, a problem with sliders, for example, is that bailing on specifying method and args as you normally would breaks the coupling that puts the slider in the correct position when you change the frame via some other pathway, like a play button.

I propose two things:

• disablemethod for updatemenus and sliders buttons/steps that leaves everything else unaffected and just skips actually executing the method so that you can hook into an event and call it yourself in a more sophisticated way. Otherwise the behavior will be unaffected.
• allow null for a method. Currently the supplydefaults enum blocks what seems like a legitimate usage when you just want a slider with no associated api call. (i.e. why have events if you can’t use them?)

/cc @etpinard @alexcjohnson @bpostlethwaite for what might need to be expedited feedback and fixes

[rreusser]

Ohhhh, I see your point. Got it. Yeah, then I like skip.

[etpinard]

as in Plotly[null], which is just a no-op

Right. But there would be no way to restyle it to that value. In other words, there are no null values in any of our attribute declarations for a reason. Actually, that’s a lie, there are few null like in contour.start but they mean this default value is auto-computed not this default value is null.