DataTable: introduce UIColumns

[Rapster]

Based on this primefaces/primefaces#10331

DataTable from PF is based on javax.faces.component.UIData, and gives the possibility to set dynamic columns with <p:columns />. The latter is not a UIColumn, and because of that, we have to copy the whole saving/restoring state algorithm from UIData (and also the visit part...)

One solution could be to use <c:foreach /> with <p:column /> but there might be few differences compare to the <p:columns />

Would it be possible just like <h:selectOneListbox /> and <f:selectItems /> do, to give the possibility to set dynamically columns (e.g <h:columns />?

That way, we could keep PF agnostic as much as we can from existing impls.