Adding PostGets to keep object graph in memory.

Jeremie found a bug in NSMenu that was crashing, this should be
fixed by this commit.   While doing this, I reviewed many classes
that had Add/Remove methods with NSObjects, and added the PostGet
methods to keep objects alive.

This should fix his bug #12529

https://bugzilla.xamarin.com/show_bug.cgi?id=12529