Save position/zoom explicitly

[davidbrochart]

Currently, every time a user pans or zooms the map, the new position and zoom are set into the shared model and eventually saved to file, but the UI doesn't react to these changes. It seems that this kind of information is rather related to the view, and each user has their own view of the map. But I agree that it can be useful to have it saved in the model too, so that it can be restored when the map is opened later.
Maybe a trade-off would be for the user to explicitly ask for it to be saved, with a button like "Save position/zoom"?

[brichet]

We discussed this with @martinRenou, and here are some ideas:

• we could use the browser local storage to keep the position and zoom for each user
• independently of saving the position in the shared document or not, the local storage will be the source of truth if it contains data
• we could possibly add some landmarks in the shared model (which would be saved/loaded manually), and use one as default if there is no data in the local storage