Allow displaying large vector data sources

[forman]

Allow displaying large vector data sources, e.g. the Glaciers CCI Shapefiles comprising several ten-thousand polygons each having thousands of coordinates.

See Vector-Data-and-FATs.

To do:

• control auto-switching to point-simplification:
  • for all features if len(data_frame) > config.num_features_max
  • feature by feature if len(data_frame) <= config.num_features_max and num_coords(feature) > config num_coords_max
• allow expanding of point simplified geometries at certain zoom level (preferred) OR toggle expanded/collapsed state by (right?)clicking. Align with Allow selecting geometries on 3D globe #489
• use a special style for feature geometries that are expandable/collapsible.