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/basic_window.py b/y2018/control_loops/python/basic_window.py
index 7caf299..7a34b3a 100644
--- a/y2018/control_loops/python/basic_window.py
+++ b/y2018/control_loops/python/basic_window.py
@@ -4,6 +4,7 @@
 from gi.repository import GLib
 from gi.repository import Gdk
 from gi.repository import GdkX11
+from color import Color, palette
 import cairo
 
 identity = cairo.Matrix()
@@ -25,6 +26,11 @@
 
 mainloop = GLib.MainLoop()
 
+def set_color(cr, color):
+    if color.a == 1.0:
+        cr.set_source_rgb(color.r, color.g, color.b)
+    else:
+        cr.set_source_rgba(color.r, color.g, color.b, color.a)
 
 def quit_main_loop(*args):
     mainloop.quit()