Fix crash in String.arg()

Fix crash in String.arg()

This commit ensures that the String.arg() function works correctly,
by registering an anonymous function with the String Prototype object
which calls the StringArg function, ensuring that the "this" object
is valid (and passing the string as an argument to StringArg instead).

Change-Id: I0a8cbaa12b39beb03a237c3ab62c6e21fafdedbf
Reviewed-on: http://codereview.qt-project.org/4385
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>