add locations next to POIs

[mfrasca]

POIs belong to locations and become visible at very deep zoom.
but how do you know where to find locations?
so I think we should add a location table, each location has coordinates and a zoom (to select the initial view), POIs must mention the location they belong to.

• while no location is selected:
  • all locations should be visible at any zoom,
• when a location is selected,
  • its zoom level and centre point are activated,
  • its POIs are loaded into the app,
  • POIs are visible at zoom 17 or deeper
  • locations are hidden at zoom 17 or deeper