Download maps to local storage for later viewing

Submitted by gli..@..ox.org

Gnome Maps lacks the ability to download maps to local storage to conserve bandwidth later (if you're on a metered mobile connection, for instance).

This is a feature that would make a user more likely to use Maps instead of opening a web browser to Google or OSM since Maps would always be available for use because it wouldn't require an always-on internet connection.
This wouldn't add much in the way of the config menu/option clutter. It could just be a button in the main window to download the map (w/ modal dialogue for radius?), and a config menu option group for when to use local data vs. fetching from OSM, and if/when to auto-update the local map.
A visible icon might be shown in the main window if the map is known to be out-of-date.