Fix various typos

Fixes various source documentation and source comment typos.

Found via: codespell -S "*.svg,*.pdf,*.po,*.pot,*.ts,*.ps,*.eps,./share/tutorials,./src/libavoid,./packaging/win32/languages,./man,./src/2geom,./share,./src/3rdparty,./doc/ChangeLog*" -L accout,andler,ba,beginin,bord,boxe,childrens,childs,clen,doub,doubleclick,ege,filetest,glight,haa,indexin,inpt,interm,ist,lew,meshe,parth,rady,ressize,ro,spile,theres,totalin,upto,yur