NestedTableColumnHeader : Make Label / setHeadersNeedUpdate() protected

In order to implement the fixedColumn property, we need to modify the Column Header so that the fixed Columns will have its header fixed and moving with him.

Therefore, I've added some code after layoutChildren but I need to have access to the height of the Label for:
final int labelHeight = (int) label.prefHeight(-1);

So maybe make it protected or add a getter.

Otherwise maybe think of adding the code directly into LayoutChildren with a special code like:
if(n.getTableColumn().impl_isFixed()){
// Do something like n.toFront(); and n.relocate(scrollX, labelHeight + snappedTopInset());
}else{
//Do normal thing
}

Issue linked with : https://javafx-jira.kenai.com/browse/RT-31647


Someone may also need to have access to "setHeadersNeedUpdate()". Because today it's called from "TableHeaderRow" but that method is package protected. So if we have a subclass of TableHeaderRow, it's impossible to call that method. So make it public or at least protected so that we can play with it in our subclass.