Add modebar buttons control into layout

[nicolaskruchten]

Let's add a layout.modebar.buttons = {add: [...], remove: [...] } attribute to make it easier for folks to force certain buttons (like the to-be-removed hovermode buttons per #5642 or the drawing controls)

[nicolaskruchten]

and change hovermode to closest -> #5639

[nicolaskruchten]

So to crisp up a bit what I think should happen in layout... We should add a new key modebar.buttons which accepts an object with keys add and remove, which should in turn accept arrays of strings. These attributes should behave the same way as the existing config keys modebarButtonsToAdd (in array-of-strings mode only!) and modebarButtonsToRemove (i.e. they should follow the same precedence rules we have right now in config, whatever they are).

The config should take precedence over whatever is set in the layout, so if the layout adds a button and the config removes it, it should be removed.

[nicolaskruchten]

(updated above to reflect the fact that we already have layout.modebar ... should this be layout.modebar.buttons.(add|remove) or layout.modebar.(buttonstoadd|buttonstoremove) ?

[archmoj]

The config should take precedence over whatever is set in the layout, so if the layout adds a button and the config removes it, it should be removed.

Hmm... shouldn't layout win over config?

[nicolaskruchten]

No, config should still win IMO

[alexcjohnson]

At some point we may want to expose an equivalent of config.modebarButtons, which would be most natural as layout.modebar.buttons = ["zoom", "pan", ...]

layout.modebar.buttons.(add|remove) is incompatible with that, unless we do something like buttons.(add|remove|all).

layout.modebar.(buttonstoadd|buttonstoremove) feels a bit long though. Would it be ambiguous to just use layout.modebar.(add|remove)?

[nicolaskruchten]

OK for layout.modebar.(add|remove). I don't want to go down the road of .buttons=[...] right now because I that's a very static view of things: you have to know all the current defaults to recreate "defaults+add-remove".