Groups and collection: create group from collection

You could still create groups as before, with Ctl + G. This will create a group
with a single visible collection.

However you can also create a group from an existing collection. Just go to
the menu you get in the outliner when clicking in a collection and pick
"Create Group".

Remember to instance the group afterwards, or link it into a new scene or file.

The group and the collection are not kept in sync afterwards. You need to manually
edit the group for further changes.

tests/python/view_layer/test_group_c.py

@@ -0,0 +1,56 @@
+# ############################################################
+# Importing - Same For All Render Layer Tests
+# ############################################################
+
+import unittest
+import os
+import sys
+
+from view_layer_common import *
+
+
+# ############################################################
+# Testing
+# ############################################################
+
+class UnitTesting(ViewLayerTesting):
+    def test_group_create_basic(self):
+        """
+        More advanced creation of group from a collection not directly linked
+        to the scene layer.
+        """
+        import bpy
+        scene = bpy.context.scene
+
+        # clean slate
+        self.cleanup_tree()
+
+        children = [bpy.data.objects.new("Child", None) for i in range(3)]
+        master_collection = scene.master_collection
+
+        grandma_scene_collection = master_collection.collections.new('Grand-Mother')
+        mom_scene_collection = grandma_scene_collection.collections.new('Mother')
+
+        grandma_scene_collection.objects.link(children[0])
+        mom_scene_collection.objects.link(children[1])
+
+        grandma_layer_collection = scene.view_layers[0].collections.link(grandma_scene_collection)
+        mom_layer_collection = grandma_layer_collection.collections[mom_scene_collection.name]
+
+        # update depsgraph
+        scene.update()
+
+        # create group
+        group = mom_layer_collection.create_group()
+
+        # update depsgraph
+        scene.update()
+
+
+# ############################################################
+# Main - Same For All Render Layer Tests
+# ############################################################
+
+if __name__ == '__main__':
+    UnitTesting._extra_arguments = setup_extra_arguments(__file__)
+    unittest.main()