This removes the global PDF_Password interface that had completely
bitrotted and moves it into the standard misc operation table with a
sensible defualt implementation.
This also cleans up and fixes the gtk frontend implementation of the
pdf password callback so it compiles.
Finally this moves the implementation to a sensible place alongside
the other exporters reducing target makefile complexity futher.
This is the initial part of a series which splits up the API interface
definitions for the frontends removing a great deal of unnecessary file
inclusion and further isolates the fronted API usage from the core
