Change how markers are stored

Now markers are stored on the map they are set on. This way it makes it possible to easily set the same marker on multiple maps.

Also the set marker functions now require you to pass the map you want the map to appear on. This helps with the point above.
Use background offsetting and change zoom

Offset the background so that the "center" of the map appears behind the logo. That way the visible portion of the background bears a resemblance to the area surrounding the user.

Also change the background zoom level logic so the background looks nicer. This is done by limiting the maximum amount of zoom the background is allowed (its currently set to the smallest scale that still displays building outlines). Finally the zoom difference has been reduced because I think it looks better.