* glib/MissingIntPtrCtorException.cs : new exception to throw if
unable to access an IntPtr ctor on a GLib.Object subclass. We need
an IntPtr ctor to be able to wrap arbitrary object handles.
* glib/Object.cs : have NativeType call LookupGType.
* glib/ObjectManager.cs : throw the new exception in a try/catch.
svn path=/trunk/gtk-sharp/; revision=27563