Add a preferences pane to the diff view

This includes to start:
+ Context
+ Column width
+ Tab size
+ Show tab indicators

Bug: Issue 3841
Change-Id: I3c8292648fd87ac2680d0eb22ce8f9e78d24429e

polygerrit-ui/BUCK

@@ -7,6 +7,7 @@ bower_components(
     '//lib/js:iron-autogrow-textarea',
     '//lib/js:iron-dropdown',
     '//lib/js:iron-input',
+    '//lib/js:iron-overlay-behavior',
     '//lib/js:iron-selector',
     '//lib/js:page',
     '//lib/js:polymer',