Allow to use GroovyDsl in FlexibleStringExpender

It will allow to call a GroovyDSL method (label, from, run service ...) directly instead of calling the Java class in flexible string expander.

The standard syntax like the first line can be replaced by the second one (from MrpScreens.xml line 64): 

`<set field="eventMessage" value="${groovy: org.apache.ofbiz.base.util.UtilProperties.getMessage('ManufacturingUiLabels', 'ManufacturingMrpRunScheduledSuccessfully', locale)}"/>`

`<set field="eventMessage" value="${groovy: label('ManufacturingUiLabels','ManufacturingMrpRunScheduledSuccessfully')}"/>`