Fix #111174: dropped nodegroups from the asset browser show selector

There was a difference between drag&drop assets from the asset browser
vs. using the search menu (in that doing it from the menu would
correctly hide the data-block selector on the nodegroup).

Since drag&drop in the Node Editor uses `NODE_OT_add_group` (not
`NODE_OT_add_group_asset` as the menu does), we have to add the hiding
here too (for this, an operator property is added which is set in
`node_group_drop_copy` if we are dropping an asset.

Alternatively, we could use `NODE_OT_add_group_asset`, too, but that
would require somehow setting the "asset" context pointer from the
dropbox copy function [how to do this wasnt obvious for me]. In that
case, we would need to set up a separate dropbox with appropriate poll
functions (so there would be one for asset groups and for the the other
groups).

@@ -372,2 +377,4 @@
 
   WM_operator_properties_id_lookup(ot, true);
+
+  PropertyRNA *prop = RNA_def_boolean(ot->srna, "is_assset", false, "Asset", "");

@@ -374,0 +380,4 @@
+  PropertyRNA *prop = RNA_def_boolean(ot->srna,
+                                      "show_datablock_in_node",
+                                      true,
+                                      "Show the datablock selector in the node (not for assets)",