* Remove O(n^2) time on DXF import
Accumulates the contents of each layer in a local Python list, then at the end assignes all the objects at once into the layer. This avoids a very slow process of traversing the objects so far and (re-)updating their properties each time a new object is added.
Fixes#16604
* Correct last change to not damage existing layer contents.
The layer could already have contents if the DXF file is being Imported rather than Opened.
The original change would lose the previous layer contents.
039dcb1346