Topology level simplify tool

[strk]

It would be very useful to have a topology-aware simplify tool. A topology aware tool would simplify only EDGES of the topology rather than full geometries. This would prevent those funny effects on crossborder lines being simplified in a different way.

I saw the topology support is already in somehow, as the node tool is able to edit multiple edges at once when moving a node.

Another alternative would be allowing for simplify tool to specify first and last point manually (more work for the user but still less than clean up after feature-level simplification)