Here is the initial issue:
- when using defaultView=latest (means latest element of the default layer)
- when map loads, we find the element, call getCenter to center the map on it
- but Polygon/Polyline needs the element to be already on the map to call
this method (at least because the map CRS is needed)
So while trying to fix that issue, I also found that using a centroid for
a complex geometry was not very friendly: the map zoom on a part of this
geometry, while it seems to be that it's better to have a full view of it.
Now that we highlight the selected element, it's also easier to get which
element is focused when there are a lot close one to the others.
This allows to known the full datalayer behaviour without needing
to load all the data, including the zoom from and to (new settings),
but also the color for example.
This will help also understanding datalayers usage and making
stats.
But no data migration is provided, it's retrocompatible (data
migration in OSM FR servers would be huge, so let's see if it's
really needed).
