Allow multiple maps incl Default map to be selected

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Is your feature request related to a problem? Please describe.
With every version of maps, except version 1.7.1, you can only select ONE single map in My Maps. Selecting a custom map disables the Default map, which has the following consequences:

• My Photos will only show photos stored in the selected custom map. Also tracks and favorites are narrowed down to what is stored in the selected custom map. But if you want to share your favorites (editable) you can only do that in a shared custom map. This in turn takes away the ability to blend in all the photos or favorites or tracks which are normally only be shown in the Default map.
• Due to a bug in 1.7.1 it seems it was possible so select ALL maps which allowed to blend in e.g. all your photos even into a custom map (which has no photos stored below).

Describe the solution you'd like
If you could select multiple maps below My Maps, especially also the Default map, this would give you the ability to create a custom map to share favorites with others and at the same time blend in your photos stored somewhere else and combining it with tracks either in the custom map or stored elsewhere.
I was really happy when I saw it working in 1.7.1, updated to 1.8.0 and it was gone again.

Describe alternatives you've considered
Alternative could be to select all sub-items of My Favorites, My Photos, My Tracks, My Devices, My Maps independently with multi selection possible in each section.

Additional context
In the end I want to share my tracks and have shared favorites with others and be able to combine it with my photos in one same view on the map.

Start by tracing the My Maps selection UI and the state that determines which map data is displayed, then compare the behavior described for versions 1.7.1 and 1.8.0. Done means users can select multiple maps, including Default, and see photos, favorites, and tracks combined across those selections; verify the alternative independent multi-selection behavior is not required.