SideBySide2: Refactor comment management to new top level class

Yank all of the code related to comment management out of SideBySide2
and into its own new top level class. This moves about 500 lines
of code from a 1800 line class to a new 400 line class, which may
make it easier to follow SideBySide2.

Change-Id: Ie24b3e3af38a9dea38a756fc5b635f1cc54f7f8a

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/ScrollSynchronizer.java

@@ -26,7 +26,7 @@ class ScrollSynchronizer {
   private LineMapper mapper;
   private ScrollCallback active;
 
-  void init(DiffTable diffTable,
+  ScrollSynchronizer(DiffTable diffTable,
       CodeMirror cmA, CodeMirror cmB,
       LineMapper mapper) {
     this.diffTable = diffTable;