optimisation of websources:
- don't look for cached image again. Done by the layer
- break retries if image found
when looking for lower zoom, only look in cache. Very important because this call is done sync
use custom queues. very important so that one layer waiting will not make another layer stuck waiting
another fix to allow creating mapView without source
refactoring to create the method updateAfterSourceChange
allow global notification for updateAfterSourceChange. Will be important for "stack source"
first version of the Stack source. Still needs some work, especially to handle async calls and "next provider" support like the android version