Decouple map sources and layers

[kylecorry31]

The layers should act almost entirely as a pass through with the hope that each source doesn't actually need a layer defined (like what is expected of plugin layers).

• Pass preferences to load method
• Look up source definitions before loading the map (will be needed to construct layers anyway) and move the following parameters there:
  • Should multiply (blend mode)
  • Is time dependent
  • Min/max zoom levels
  • Refresh interval
  • Refresh broadcasts (for below beacon/path/navigation decoupling)
  • Cache key format string (with preferences as format args?) (for below cache key decoupling)
• Pass context to sources
• Determine how to decouple beacon/path/navigation layers from the data updates (maybe specify broadcasts it listens to, just like widgets)
• Layer level preferences (move into GeoJsonLayer/TileMapLayer):
  • Show labels
  • Path background color
• Photo map layer should pass ID along as preference
• Determine how to construct cache key for tiles (maybe just move that to the source) or have the source definition have a cache key format string
• Make MyLocationGeoJsonSource responsible for its own bitmap creation
• Tools register sources rather than layers (MapLayerDefinition -> MapSourceDefinition and layers are automatically constructed)
• Update plugin documentation
• Consider replacing open feature with a tool action invocation (ex. invoke action, passes along ID)
• Replace source definition create with class reference (no parameters should be needed)