Allow to manage map backgrounds

[offtherailz]

Description

Many times the user needs to add or remove a background to the maps. Now the only way to do it is to edit the map configuration manually.

We should allow to manage backgrounds of maps using MapStore UI.

Proposed features (draft):

• Add backgrounds
  • from catalog
  • From a list of available tile providers (OSM/Google/Bing...). The list should show a thumbnail of each background available, maybe grouped by category
• Edit current backgrounds
  • Remove
  • Customize (if it comes to from a catalog service)
    • Thumbnail
    • Label
    • Format
    • Style
    • Custom params
• Reset to defaults (replacing current backgrounds with ones from new.json. This may be useful to quickly change all backgrounds to many maps).

Other useful information (optional):

See also #3170

Mockup available here that includes also the part related to #3170

Some evaluation have been already performed by @allyoucanmap for the thumbnails management here

This issue does not includes the implementation of the support for the catalogs basic auth, that work is part of another issues not connected now #3380.