I think I've changed all corresponding appearances from FT_STRING to FT_GUID, so assert the FT_ type as it should only be a FT_GUID now.

Add a generic implementation in guid_utils.h to have a way to store data about GUID to name resolving (something like value_string for e.g. int). It might be better to have a single registry for all GUID's of all dissectors and implement the GUID name resolving into the proto_tree_add... functions.

svn path=/trunk/; revision=18935