Add support for Paragraph and Page borders.

Note: Typing "---" in MS Word and pressing Enter inserts bottom paragraph border with special size (sz="6" in the docx format).

Workaround for inserting visually similar element:

Add a horizontal line using a table with a single border. Here is some code demonstrating how to do this:
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var document = new RadDocument();
var editor = new RadDocumentEditor(document);

Table table = new Table();
var topBorder = new Telerik.Windows.Documents.Model.Border(1, BorderStyle.Single, Colors.Red);
var borders = new TableBorders(table.Borders.Left, topBorder, table.Borders.Right, table.Borders.Bottom);
table.Borders = borders;

var row = new TableRow();
var dummyCell = new TableCell();
var dummyParagraph = new Paragraph();
dummyParagraph.FontSize = 0;
dummyParagraph.LineSpacing = 0;
dummyParagraph.SpacingAfter = 0;
dummyParagraph.SpacingBefore = 0;
dummyCell.Blocks.Add(dummyParagraph);
row.Cells.Add(dummyCell);
table.Rows.Add(row);

editor.InsertTable(table, shouldInsertParagraphBeforeTable: true);            
this.rtb.Document = document;
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