scattermapbox SymbolSelector to accept a string

[dmt0]

The following is a spec for marker.symbol for scattermapbox trace:

Type: (string or array of strings)
Default: circle
Sets the marker symbol. Full list: https://www.mapbox.com/maki-icons/ Note that the array marker.color and marker.size are only available for circle symbols.

Currently we only have a dropbox with predefined symbols