Disable "chat history size" box if the "Show history" checkbox is false.

src/net/java/sip/communicator/plugin/generalconfig/GeneralConfigurationForm.java

@@ -242,6 +242,11 @@ private void initDefaults()
             historySizeSpinner.setEnabled(false);
         }
 
+        if (!ConfigurationManager.isHistoryShown())
+        {
+            historySizeSpinner.setEnabled(false);
+        }
+
         sendMessageComboBox.setSelectedItem(
             ConfigurationManager.getSendMessageCommand());