Added ui-reflow-headertext class to DataTable for Reflow mode

[mertsincan]

On reflow mode, the header title of columns are very difficult to find in the nested structures/components in f:facet. Therefore, users can use ui-reflow-headertext class.

Test code;

<p:column ...>
   <f:facet name="header">
         <h:panelGroup>
            <h:outputText value="TEST TEST" styleClass="ui-reflow-headertext"/>
         </h:panelGroup>
         <h:selectOneMenu>
            ...
          </h:selectOneMenu>
   </f:facet>
...
</p:column>