Add tls-interaction property to SoupSession

This can be used by applications to do client-side certificates via
the new g_tls_interaction_request_certificate().  Will be used by
OSTree at least.

Also add a test to tls-test.

Based on a patch from Colin Walters.

https://bugzilla.gnome.org/show_bug.cgi?id=334021
Pop the thread default main context not from the callback but from where it was pushed

Based on a patch by Dmitry Shatrov <shatrov@gmail.com>, see
http://mail.gnome.org/archives/libsoup-list/2011-November/msg00000.html

https://bugzilla.gnome.org/show_bug.cgi?id=729737
Updated Brazilian Portuguese translation
Updated Spanish translation