Symmetric encryption in seahorse-nautilus

Originally created by @intrigeri on #6607 (Redmine)

Symmetric encryption was added upstream. It needs:

gpgme1.0>=1.4.2 (from wheezy-backports), otherwise there is a bug
seahorse-nautilus 3.10.1 (#7429 (closed), http://bugs.debian.org/734983), or a backport of the relevant commits:
- https://bugzilla.gnome.org/show_bug.cgi?id=325803 (commit ae25bf9): Add support for symmetric encryption in seahorse-tool
libcryptui 3.10.0+ (#7430 (closed), http://bugs.debian.org/734984), or a backport of the relevant commits:
- https://bugzilla.gnome.org/show_bug.cgi?id=325803
- https://bugzilla.gnome.org/show_bug.cgi?id=699161
- Commit cd74aa6: Add symmetric encryption as a choice in “choose recipients” dialog
- Commit c744886: Add cryptui_need_to_get_keys_or_symmetric()
- Commit f110e1c: Avoid repeating the self encryption key in cryptui_key_chooser_get_recipients()
- Commit 6012bfe: Add support for symmetric encryption

Edited May 15, 2020 by intrigeri

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