python: added ability to move multiline statements

fixed PY-6587 Move Statements: inconsistent selection on moving more than one statement up or down.

python/testData/mover/multiLineSelection5_afterDown.py

@@ -0,0 +1,8 @@
+class Test(object):
+
+    def q(self):
+        c = 3
+
+<selection><caret>a = 1
+b = 2
+</selection>