Misc. documentation and source comment typo fixes
Found via `codespell -q 3 -L ,ang,ba,childs,eiter,forse,objext,pevent,shouldbe,thru,uint,unselect,vertexes --skip="./synfig-studio/po,./synfig-core/po,/synfig-core/m4,./synfig-osx/,./gtkmm-osx,./bugs,.*.eps,*.sif"`