When a user recovers a page from browser history where placed a
marker in different map pane (visible map layer) marker was
successfully added to the map but the map center is the one
defined at Settings map properties so the marker was not visible
to the user.
Now when map_location form has valid coordinates we use them
instead of default map center settings. This will avoid the user to
have to rellocate the marker (or find the correct pane where the
marker was added) if already placed.
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