Set preferred viewer for base map

[na9da]

Split from #7626
Required for #6845
Merge after #7710

What this PR does

• Adds trait preferredViewer to mappable mixin
• Automatically switch to preferred viewer when changing base maps
• Automatically switch base map to a compatible one when changing the viewer mode

Example base map config (full example):

"baseMaps": {
    "items": [
      {
        "item": {
          "id": "basemap-bing-aerial-with-labels",
          "preferredViewerMode": "2d"
        },
        "image": "build/TerriaJS/images/bing-aerial-labels.png"
      },
      {
        "item": {
          "id": "basemap-bing-aerial",
          "preferredViewerMode": "2d"
        },
        "image": "build/TerriaJS/images/bing-aerial.png"
      }
    ]
  }

Test me

• Open this CI link with both aerial Bing maps configured to prefer 2d mode
• Switch from OSM to any bing map
• Observe that the viewer mode is switched to 2d and a toast notification is shown
• Switch viewer mode to 3d, observe that the base map switches to OSM and a toast notification is shown

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]

Tracks the currently loading base map and sets it so that the last selected base map gets applied. Previously if the user selected multiple base maps then it would be applied in the order they were loaded instead of the order they were selected. This can be surprising to the user when some base maps are slow to load.