Allow the customers to work with Shape objects.

For all characters which cannot be represented in XML as defined by the XML 1.0 specification, the characters are escaped using the Unicode numerical character representation escape character format _xHHHH_, where H
represents a hexadecimal character in the character's value. This is dedined in the OpenXML specification in 22.9.2.19 ST_Xstring (Escaped String).

Currently this causes the following issues:

- Documents conaining such characters are imported incorrectly, e.g. <si><t>_x001B_</t></si> is imported as the plain text _x001B_

- ArgumentException with clarification similar to "' ', hexadecimal value 0x1B, is an invalid character." is thrown when trying to export such documents. Example code to reproduce:

            Workbook workbook = new Workbook();
            Worksheet worksheet = workbook.Worksheets.Add();

            CellSelection selection = worksheet.Cells[1, 1]; //B2 cell
            selection.SetValue(string.Format("Error Value: {0} ", Encoding.ASCII.GetString(new byte[] { 27 })));

            string fileName = @"D:\temp\SampleFile.xlsx";
            IWorkbookFormatProvider formatProvider = new XlsxFormatProvider();
            using (FileStream output = new FileStream(fileName, FileMode.Create))
            {
                formatProvider.Export(workbook, output);
            }

The way the RadSpreadProcessing decides if a row should be exported works slow. It iterates through all the rows in the UsedCellRange and checks if some property is set and if not, checks if some cell is set. This is slow because each row is checked one by one.

If shape extent is declared, the two cell anchor points for the drawing element are ignored. For example if the TwoCellAnchorElement's from and two points declare that the image should be anchored starting from cell A1to F10 and further in the drawing declaration there is defined extents element with image size values, anchor element's points are ignored. 

For example, if a user wants to set a 5x5 array of values in the worksheet he has to iterate through all the values one by one and set it to the worksheet. With that API the user should be able to set the entire array with just one method call giving it the array.