Closes#7559

Fixes issue where settinglayout.legend3.visible tofalse caused bothlegend andlegend3 to disappear.

Also adds a Jasmine test to prevent regression.

It seems this bug was becauselegend._id was not being added to the processed legend object during thedefaults step ifvisible was false. This DID cause the legend not to be drawn (desirable), but it had the side effect of causing the plot step to "assume" the legend_id to belegend if not available, which caused the visibility setting to also be applied to the first legend (undesirable).

Note that the onlyactual logic change is on line 242 ofsrc/components/legend/defaults.js; the other changes in that file are just auto-formatting changes.

To test

• Open@my-tien 's codepen linked in[BUG]: legend3.visible = false affects visibility of other legend #7559
• Replace the Plotly.js CDN link with the link to the CircleCI build for this branch:https://output.circle-artifacts.com/output/job/aff93db6-865a-4e06-a4c6-816483ea13b4/artifacts/0/dist/plotly.js
• In the plot definition, changelayout.legend3.visible fromtrue tofalse and make sure that neither of the other two legends disappear. (legend3 is not visible in any case becauseshowlegend is set tofalse at the trace level and there's only one trace in that legend)