Change leaflet map background color

[na9da]

Split from #7626
Required for #6845

What this PR does

Adds backgroundColor trait to base maps for setting the background color of the Leaflet map container when the base map is selected.

Example config:

{
  "baseMaps": {
    "items": [
      {
        "item": {
          "id": "basemap-bing-aerial-with-labels"
        },
        "backgroundColor": "#00101d"
      },
      {
        "item": {
          "id": "basemap-bing-aerial"
        },
        "backgroundColor": "#00101d"
      },
      {
        "item": {
          "id": "basemap-bing-roads"
        },
        "backgroundColor": "#acc7f2"
      }
    ]
  }
}

Test me

• Open this CI link which uses this catalog definition
• Switch to 2D mode
• Switch between the Bing base maps
• Observe that the background color of the map container changes with the base map and is different to the default grey color

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.

[na9da]

Actual (non-prettier) change is this line which updates all traits from the new definition instead of just the image trait.

[micholm]

LGTM! Great feature :)