Add basemap model_id to Map._layer_ids attribute

[giswqs]

When initializing the Map with a basemap, the basemap model_id is not added to the Map._layer_ids attribute, resulting in an error when trying to removing the basemap from the map. This PR fixes the bug by adding the basemap layer id to the Map._layer_ids attribute

import ipyleaflet
m = ipyleaflet.Map()
m
m.remove(m.layers[0])

[martinRenou]

Thanks!