Use direct assignment for string in class initialization

    
Change string initialization to use direct assignment instead of
a class method in the GParted_Core class initialization.

Thanks go to Jan Claeys for pointing out this potential problem
and providing the following link:
https://bugzilla.gnome.org/show_bug.cgi?id=627724#c7

This link contains a comment by Matthias Clasen that says:

    "In a class init function, don't do anything that causes
    other threads to initialize classed types."