Possibility to use more that one custom layer on ID editor

[dp7x]

Hello I wanted to propose a new functionality that would be very useful for me when mapping, and I found this old post with my same idea. Are there any news about this ? It's not fundemental for me to save the custom layers in user's profile: it would be enough to me to have 2 or 3 custom layers options, to set up each time I log in pasting the URL. Then, during the mapping work, it woule let me to switch very fast from a layer to another. Thanks !

        More generally, it would be convenient if we could add as many custom layers as we wish: in France there are at least 3 rendering that might be useful at the same time (IGN aerial photos, cf #3420, Strava heatmap and [BANO](http://tile.openstreetmap.fr/~cquest/leaflet/bano.html)). 

It would mean a bit of UI reworking, but maybe something like that?

Originally posted by @bagage in https://github.com/openstreetmap/iD/issues/2095#issuecomment-281755531