The 'save over' popup was only appearing based on a string comparison of the operator name ("Save"). Changed this to use a hidden operator property: "check_existing". Python operators must
have this property for the file selector confirmation too.
This property can also be set to false, to prevent checking for existing files, useful in the File->Save menu item to prevent the dangerously missable confirmation popup.
- use xml.sax.saxutils.escape() to give an XML compatible string.
- in some cases material indicies could be invalid. use the default material rather then throwing an error.
