Add a default basemap

[bazmap]

It would be great to have a default basemap just to localise our data.

The basemap can be OpenStreetMap (https://tile.openstreetmap.org/${z}/${x}/${y}.png) for example.
The basemap need to stay away from the MVT description, it wil not be in the json exported, it is only here to help the user to localise his data on the map and check the zoom level.

It always add a basemap that I need to drop befor exporting the json thus making the process to import+change+export more time consumming.