MultipleChoice class has a d-pointer and doesn't crash!

MultipleChoice class has a d-pointer and doesn't crash!
Frederik: mine crashed on the first attempt also,  I think
the copy constructor is the key.  If we don't provide one,
c++ just generates one that copies all member variables.
Which causes d-pointer to be deleted when the first object
is destructed :)

svn path=/trunk/KDE/kdeedu/libkdeedu/; revision=690896