Fixes#16550.
Fixes#16552.
* Layer blocks receive a name with the "LayerBlock_" prefix.
* They are put in a layer if the "Use layer" import option is checked.
* If Draft or Sketch objects are created a Part_Compound instead of a Draft_Block is created. This was done because a Draft_Block does not claim its children. I am not sure about this change. Maybe the behavior of the Draft_Block should be changed instead.
@yorikvanhavre: do you know why a Draft_Block does not claim its children?
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