Fix misc. source comment and doxygen typos in ./toonz/sources/toonzlib (#2740)
* Fix misc. source comment and doxygen typos in ./toonz/sources/toonzlib
Found via `codespell -q 3 -S *.ts,./thirdparty -L dum,inbetween,sinc,uint ./toonz/sources/toonzlib`