restructure public/local layers

[Gjum]

• multiple sources, which can be editable/frozen
• current visible selection references ids/urls
• editing frozen layer/feature imports layer to local layers and hides remote layer
• remove visibility flag from layers?
  • useful for sending optional layers
  • bad surprises: sharing hidden private layer by accident

[Gjum]

decided to replace the layer functionality with filters #30

[Gjum]

reconsidering using this abstraction instead of #30 because of its simplicity