[gcs] 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.
[maps] add support for multiple resolution tiles

If available in cache, the highest resolution is displayed.
Lower resolution are displayed behind higher resolution.
TODO: change the max resolution for download at runtime in gcs (startup
parameter for now)

This should fix #277 and close #364
[gcs] minor change on load tile text
[maps] add MapQuest OSM and Open Aerial sources
[zoom] fix zoom level

bound tiles to maps_zoom (cache or http)
if lower res available in cache try http according to policy
Merge branch 'maps_zoom_level'

GCS: basic support for higher resolution background maps
* specify max map zoom level with -maps_zoom option
  - defaults to 18, max is 22
* Only a startup parameter for now, not changeable at runtime.
* If only lower res tile cache, try http according to policy.
* Lower resolution are displayed behind higher resolution.
[supervision] don't launch sitl if popup was canceled