Anti-aliasing in xlsgen 4.0

xlsgen 4.0 introduces anti-aliasing for rendering charts. Charts are arguably much nicer.

There are 3 anti-aliasing levels to choose from. xlsgen 4.0 defaults to anti-aliasing level 1. Older xlsgen versions did not have anti-aliasing at all.

workbook.AntiAliasing = xlsgen::antialiasing_level_1;