HACKING: Recommend Glade 3.8.5, add link for fishman ctags fork

Also note that Glade callbacks don't need to go in `callbacks.c` anymore.
While I agree, it becomes more subtle with #358, as if we indeed connect the signals manually (through generated code), the connection code has to locate the function.  Of course it could be listed in the others module header and that header included in callbacks.c.