UiCommand: support confirmation message

For some commands, like "Delete-Project", a confirmation dialog must be shown.
Add a support for it in UiCommand. If confirmation message is null or empty
no confirmation dialog is shown. If user cancel confirmation dialog, no
action is triggered.

Change-Id: I2b1e4759b488a3b5079fdb54c6c862ab59397ddf

gerrit-common/src/main/java/com/google/gerrit/common/data/UiCommandDetail.java

@@ -21,4 +21,5 @@ public class UiCommandDetail {
   public String label;
   public String title;
   public boolean enabled;
+  public String confirmationMessage;
 }