Combine Add base layer et Import Geojson layer

[Bakaniko]

In the 2 cases mentionned, it is a matter to import shape through Geojson files.

• Add base layer: via an url
• Import Geojson layer: via a file on the computer

I think we can simplify the interface with only on button but in the dialog box, let the user choose the way to load the data.

Plus give some reminders :

• geojson file only
• 4326 projection