Add and use capability of defined individual map types zoom minimum and maximum levels.

Add and use capability of defined individual map types zoom minimum and maximum levels.

Various tile servers have differing zoom extents.
Thus don't need to generate download requests that we know are going to fail.
The zoom levels can be customized for new maps.

NB this only caters for zoom levels that don't vary across the world.
It is possible some tile servers support differing zoom levels in different regions.