Fix bug: Spell error "=" should be "=="
Fix Clang warning: trigraph ignored [-Wtrigraphs]

??/ is a c++ trigraphs, so escape one ?
Fix clang warning: comparison of two values with different enumeration types
Fix warning: unused variable
There is no need to have a virtual member for Worksheet at present
Add worksheet dimenstion access API

first/last Row/Column
Fix memery leak of QXlsx::Cell
Add data validation read/write support
Using QXlsx::CellRange instead of the private XlsxCellRange
Export the dimension api of worksheet