#21 Added support for external xmi.id generator

[Max Zinal]

xmi.id's are generated by MDFOuterMostPackageImpl. We
use the NSMDF to edit models which are fragments of a
bigger single model. And so we really need to make
xmi.id's for newly created instances more unique then
they currently are.

So this patch adds interface MDFXMIIdGenerator which
the user can implement, setter and getter methods to
make the instance of the user's generator to be used.

[Max Zinal]

Two patchfiles and one new file