* generator/CallbackGen.cs : switch to NativeCallbackSignature.
* generator/GenBase.cs : add NativeCallbackType member.
* generator/IGeneratable.cs : add NativeCallbackType member.
* generator/ManagedCallString.cs : add guarded post call struct
marshaling back to the native struct.
* generator/NativeCallbackSignature.cs : new parallel to ImportSignature
but using NativeCallbackType instead of MarshalType.
* generator/Signal.cs : switch vm and sig marshaler callbacks to
NativeCallbackSignature. Perform guarding post call struct marshaling
back to the native struct.
* generator/Parameters.cs : add NativeCallbackType member.
* generator/SimpleBase.cs : add NativeCallbackType member.
* generator/StructBase.cs : add NativeCallbackType member using IntPtr
to support NULL handling.
* gtk/NodeCellDataFunc.cs : update native marshaler sig.
svn path=/trunk/gtk-sharp/; revision=76011
