Wordprocessing: Add explicit properties to Table

[m-gallesio]

DocumentFormat.OpenXml.Wordprocessing.Table should provide properties to explicitly access some of its fixed fields, like TableProperties, TableGrid and its collection of TableRows. This would match how several other classes already work (Paragraph.ParagraphProperties, Run.RunProperties, ...). TableRow might also appreciate a way to explicitly access its cells, unless .Descendants<TableCell>() is deemed enough.