configurable higher maps zoom level

Attempt to solve #277 

specify max map zoom level with -maps_zoom option
- defaults to 18, max is 22
  - TODO: check that a valid zoomlevel was given
  - add it to the menu to change at runtime?
- if available, larger (low zoomlevel) tiles in cache are loaded right away,
  even if higher zoomlevel maps could be downloaded.

Also if you have a high zoom level, getting the tiles takes ages, because that is not parallelized...