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_d.py

@@ -0,0 +1,76 @@
+# ############################################################
+# 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_write_load(self):
+        """
+        See if saving/loading is working for groups
+        """
+        import bpy
+        scene = bpy.context.scene
+        layer_collection = bpy.context.layer_collection
+
+        while len(scene.view_layers) > 1:
+            scene.view_layers.remove(scene.view_layers[1])
+
+        # create group
+        group = layer_collection.create_group()
+
+        self.assertEqual(1, len(bpy.data.groups))
+        self.assertEqual(1, bpy.data.groups[0].users)
+        self.assertEqual(3, len(bpy.data.groups[0].objects))
+
+        import os
+        import tempfile
+        with tempfile.TemporaryDirectory() as dirpath:
+            filepath = os.path.join(dirpath, 'layers.blend')
+
+            for i in range(3):
+                # save and re-open file
+                bpy.ops.wm.save_mainfile('EXEC_DEFAULT', filepath=filepath)
+                bpy.ops.wm.open_mainfile('EXEC_DEFAULT', filepath=filepath)
+
+                self.assertEqual(1, len(bpy.data.groups))
+                self.assertEqual(1, bpy.data.groups[0].users)
+                self.assertEqual(3, len(bpy.data.groups[0].objects))
+
+
+            # empty the group of objects
+            group = bpy.data.groups[0]
+            while group.objects:
+                group.view_layer.collections[0].collection.objects.unlink(group.objects[0])
+
+            # save and re-open file
+            bpy.ops.wm.save_mainfile('EXEC_DEFAULT', filepath=filepath)
+            bpy.ops.wm.open_mainfile('EXEC_DEFAULT', filepath=filepath)
+
+            self.assertEqual(1, len(bpy.data.groups))
+            self.assertEqual(0, bpy.data.groups[0].users)
+            self.assertEqual(0, len(bpy.data.groups[0].objects))
+
+            # save and re-open file
+            bpy.ops.wm.save_mainfile('EXEC_DEFAULT', filepath=filepath)
+            bpy.ops.wm.open_mainfile('EXEC_DEFAULT', filepath=filepath)
+
+            self.assertEqual(0, len(bpy.data.groups))
+
+
+# ############################################################
+# Main - Same For All Render Layer Tests
+# ############################################################
+
+if __name__ == '__main__':
+    UnitTesting._extra_arguments = setup_extra_arguments(__file__)
+    unittest.main()