Add path_edit.py to bazel BUILD

Add the path_edit tool to bazel. Abstract out the shared base_window
class used for path_edit and graph_edit.

Change-Id: I8c3d493cfa7fc954e39da82210cf1cb7ec880b13
diff --git a/y2018/control_loops/python/path_edit.py b/y2018/control_loops/python/path_edit.py
index 885b36c..ce2118a 100644
--- a/y2018/control_loops/python/path_edit.py
+++ b/y2018/control_loops/python/path_edit.py
@@ -1,3 +1,5 @@
+#!/usr/bin/python3

+

 from __future__ import print_function

 import os

 import basic_window