Add color class and a dict of common colors.

Add a color class and a dictionary of commonly used colors so that we
can use set_color() instead of calling the set_source_rgb() function
with rgb values every time we need to change colors. Also, removed an
unused set_color() function from within graph_edit.py.

Change-Id: I113285f1afcb11cfc3a236bedeb5b885da19eaa9
diff --git a/y2018/control_loops/python/BUILD b/y2018/control_loops/python/BUILD
index 2335897..43fa3a8 100644
--- a/y2018/control_loops/python/BUILD
+++ b/y2018/control_loops/python/BUILD
@@ -167,6 +167,7 @@
     name = "basic_window",
     srcs = [
         "basic_window.py",
+        "color.py",
     ],
     restricted_to = ["//tools:k8"],
     deps = [