[evince] Adding support for viewing signatures

[Vasco Dias <contact vascodias me>]

 Hi there,

I'm doing an internship and my objective is to add basic visual information to
Evince
for showing signatures status in documents that are signed.

My mentor is adding that support to poppler and in his internal code repository
he already
has most of that done. At least he can already sign documents and at least
verify if the
signature is valid or not. Some features are still lacking and he is working on
it.

My task, like I said, it to take care of the gui interface, so Evince.
The focus was to add something similar to what acrobat reader shows, so a
sidebar and
populate a tree view with relevant information.

Since I'm new with developing using Gtk and the related technologies I'd like
for someone
to take a look for now at the repository at github where I'm saving my changes
and if
someone spots something strange or has suggestions to please tell me.

In the future when I'm happy with what I'm hacking I'd like to push it upstream
as my mentor
will also try to push the changes to poppler to upstream.

Notes:
1) Right now the current code tries to use just 2 methods not yet available in
official poppler,
but just to test the gui you can remove those invocations and it should compile
and work fine.

Signatures support for this start at line 3321 in backend/pdf/ev-poppler.cc,
just change and
put some hardcoded values there ^^

2) With this changes we are just making poppler depend on openssl and Evince
won't have to depend
on anything extra.

Links:
Bug link: Verifying digital signatures -
https://bugzilla.gnome.org/show_bug.cgi?id=614929
My Code at Github: https://github.com/diaz/evince-signatures
A screenshot: http://ompldr.org/vZjRocA

Thank you for your time,
Vasco Dias