Allow multiple "Custom background layers" (or add more predefined layers?)

[Binnette]

Is your feature request related to a problem? Please describe. No, It's a feature request.

Describe the solution you'd like Currently uMap allow users to add only 1 unique "custom background layer":

Instead, I would like to add more custom background layer. That way, the user can switch between them.

Describe alternatives you've considered Otherwise we could add more "background layers" in the list of predefined layers:

For example, I am missing those layers:

• https://{s}.tile.osm.ch/switzerland/{z}/{x}/{y}.png
• https://maps.refuges.info/hiking/%z/%x/%y.png
• May be some other on wiki