Convert OpenPGP Applet into a proper GNOME Shell extension

Originally created by @intrigeri on #8310 (Redmine)

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

Attachments

• tray-hidden.png
• tray-displayed.png

Parent Task: #8309 (closed)

Subtasks

• #11934 (closed)
• #11935 (closed)
• #11936 (closed)

Related issues

• Related to #7433 (closed)
• Related to #16415 (closed)
• Related to #16531 (closed)
• Has duplicate #14925 (closed)
• Blocked by #7711 (closed)
• Blocked by #16608 (closed)
• Blocked by #16709 (closed)