Additional background maps

[hauke96]

Additional background layer would be nice. A button in the toolbar should open a simple list on which the user can choose one of many background maps.

Layers

These layers would be nice:

• Bing imagery (see here under "Bing Maps")
• ESRI imagery (use the XYZ-tiles from https://server.arcgisonline.com/arcgis/rest/services/World_Imagery/MapServer/tile/{zoom}/{y}/{x})
• Other aerial imagery like Sentinel
• Maybe OpenTopoMap

Other layers if available and useful.

Overlays

Overlay layers are not part of this feature, they come later in #103 .