Have org.Tails.OpenPGPApplet send signals when the type of what's in the clipboard changes

This will allow the OpenPGP applet’s GNOME Shell extension to update its icon and menu dynamically just like the current Perl applet does.

Feature Branch: intrigeri/openpgp-applet:feature/8310-gnome-shell

Parent Task: #8310

Original created by @intrigeri on 11934 (Redmine)

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