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.
The die() API for abnormal termination does not belong within the core
of netsurf and instead errors are propogated back to the callers.
This is the final part of this change and the API is now only used within
some parts of the frontends
