Initialize common KeyCodes in NavigationTable

Refactor common KeyCode initialization into NavigationTable.
This reduces some of the KeyCode initialization proliferation,
and ensures that similar things are done using similar keys.
It also drastically reduces the constants needed.

Change-Id: I0c88b3dcbe332e02b415a9ef222744588ea0a7b2

gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeConstants.java

@@ -45,9 +45,7 @@ public interface ChangeConstants extends Constants {
   String changeTableColumnLastUpdate();
   String changeTableNone();
 
-  String changeTablePrev();
-  String changeTableNext();
-  String changeTableOpen();
+  String changeItemHelp();
   String changeTableStar();
   String changeTablePagePrev();
   String changeTablePageNext();