Do not react to all Scene changes in LayerView but just childrenChanged
Do not react to all Scene changes in LayerView but just childrenChanged

When the layer data is updated, a new node is attached to the scene
root. We can thus watch for only children changes since everything else
can be ignored.

Contributes to CURA-938

hmflash/Cura,hmflash/Cura,fieldOfView/Cura,Curahelper/Cura,fieldOfView/Cura,senttech/Cura,totalretribution/Cura,ynotstartups/Wanhao,ynotstartups/Wanhao,senttech/Cura,Curahelper/Cura,totalretribution/Cura