Anim: make RNA property bone_collection.parent writable

Moving a bone collection to another parent is now possible in Python by
assigning to `bone_collection.parent`.

Thanks to Sergey for the implementation.

tests/python/bl_animation_armature.py

@@ -111,6 +111,37 @@ class BoneCollectionTest(unittest.TestCase):
         # Check the array order.
         self.assertEqual([root1, r1_child1, root2, r1_child1_001, r2_child1, r2_child2], list(bcolls.all))
 
+    def test_parent_property(self):
+        # Just to keep the rest of the code shorter.
+        bcolls = self.arm.collections
+
+        self.assertEqual([], list(bcolls), "By default an Armature should have no collections")
+
+        # Build a hierarchy.
+        root1 = bcolls.new('root1')
+        r1_child1 = bcolls.new('r1_child1', parent=root1)
+        r1_child2 = bcolls.new('r1_child2', parent=root1)
+        root2 = bcolls.new('root2')
+        r2_child1 = bcolls.new('r2_child1', parent=root2)
+        r2_child2 = bcolls.new('r2_child2', parent=root2)
+
+        # Check getting the parent.
+        self.assertEqual(root1, r1_child1.parent)
+        self.assertEqual(root1, r1_child2.parent)
+        self.assertEqual(root2, r2_child1.parent)
+        self.assertEqual(root2, r2_child2.parent)
+
+        # Move r1_child1 to be a child of root2 by assigning the parent.
+        r1_child1.parent = root2
+        self.assertEqual(root2, r1_child1.parent)
+
+        # Check the sibling order.
+        self.assertEqual([r2_child1, r2_child2, r1_child1], list(root2.children))
+
+        # Make r1_child1 a root.
+        r1_child1.parent = None
+        self.assertIsNone(r1_child1.parent)
+
     def test_bone_collection_bones(self):
         # Build a hierarchy on the armature.
         bcolls = self.arm.collections