A slightly modified patch from Leon for <<Make "showDescription" of <lookup> filed configurable individually>> https://issues.apache.org/jira/browse/OFBIZ-5211

For now, It's configured globally through "widget.lookup.showDescription" in widget.properties. 
In some case, we want to disable this feature for particular <lookup> filed. So we try to make it enabled.

1. update the widget-form.xsd to add new attribute "show-description" to <lookup> filed
  * if set this attribute explicitly to true or false, then showDescription feature is enabled or disabled respectively
  * if this attribute is not set (default), then showDescription or not depends on the "widget.lookup.showDescription" setting
2. modify ModelFormField and MacroFormRenderer to make it works
3. Since we introduce individual "showDescription" configuration, the global "SHOW_DESCRIPTION" was unable to work properly. It has been replaced by dynamic values
4. Show description function depends on ajax-autocompleter, but not vice versa. That's why the condition [options.showDescription != ""] was removed from _createAjaxAutoComplete function.

jleroux: I formatted the XSD documentation and used 

showDescription = "Y".equals(UtilProperties.getPropertyValue("widget", "widget.lookup.showDescription", "Y"));
instead of
showDescription = "Y".equals(UtilProperties.getPropertyValue("widget", "widget.lookup.showDescription", "N"));

Because, in widget.properties, widget.lookup.showDescription is Y by default (and I know that well, since I made this description working correctly in all cases)

git-svn-id: https://svn.apache.org/repos/asf/ofbiz/trunk@1491936 13f79535-47bb-0310-9956-ffa450edef68