XMLWriterSimple : #set_xml_writer and #close are changed to open file in close method.
remove Packager#add_workbook method.
rename PrintStyle to PageSetup
add Worksheets class.
format.rb : simplize Format#get_xf_index
move method from Worksheet to PageSetup
add Workbook#style_properties
move method from Packager to Worksheets.
refactoring Format#get_dxf_index
remove @sheetnames and #get_sheet_index from Workbook, and add Worksheets#index_by_name
move method related to sheetname from Workbook to Worksheets.
add Formats class and use parameter for Format#new.
class Worksheets is renamed to Sheets. and add external_links method to both Worksheet and Chartsheet.
remove Worksheet#xf because it isn't used.
add class DataValidation.
add :comments to Worksheet's attr_reader.
move several method from Worksheet to Sparkline.
add constant to Worksheet.
Worksheet : remove temporary variable 'attributes' if possible
use Writexlsx::OFFICE_URL in Workbook and Worksheet.