Store unordered_map by value
Store unordered_map by value

This way you can request the keys by value as well. It's probably less efficient but easier to use.

Contributes to issue CURA-4017.

Ultimaker/CuraEngine,ROBO3D/CuraEngine,ROBO3D/CuraEngine,ROBO3D/CuraEngine,Ultimaker/CuraEngine