Allow multiple colorbars per trace

[etpinard]

Currently only one colorbar is allowed per trace even though multiple colorscales can be declared (e.g. marker.color and marker.line.color and even line.color for scatter3d).

[etpinard]

Might be nice to get this one in during the #3431 push.

[etpinard]

The scatter3d line.color + marker.color part was done in #3803

At present, marker.line colorscales still doesn't support color bars. Please comment if someone would like this feature added.

[etpinard]

Closing. Someone should open another issue if ever marker.line color bars get requested.