Parser: Store imported key filename in Parsed & Parser

1. Access to the filename is useful in many ways. Importers, e.g.
GcrOpenSshImporter, have access to the GcrParsed, and can read the
filename to preserve filenames of imported keys.

2. When the GcrViewerWindow reads a key to create a parser, store
the filename as an instance variable of the parser

3. In the parse_...() functions where the GcrParsed is created, store
the filename as an instance variable via push_parsed (read it from
the parser).

https://bugzilla.gnome.org/show_bug.cgi?id=705225