editsvg.js: rename childs to children

for linguistic reasons
svgedit: Rename 'url' property to 'textedit'

This is more explicit
svgedit: remove roundPos()

round() is more generic, roundPos() is replaced everywhere by now.
svgedit: Improve Point (precision)

add a precision property to Point() class and
use round() instead of getRoundDiffPos() (making that obsolete)
svgedit.js: Code comment cleanup
svgedit.js: Remove obsolete getRoundDiffPos()