add more ways to trigger detection of duplicate contacts functionality

Forwarding and summarising one half of these two issues:

https://bugzilla.gnome.org/show_bug.cgi?id=549609 https://bugzilla.gnome.org/show_bug.cgi?id=269482

The detection of duplicates amongst multiple contacts can be a manual process by a human or a automatic process. The automatic process is already implemented but the only way to trigger it is to copy/move a contact between address books. An actions menu item, a keystroke and a context menu item called "Detect duplicates" could be added to search for duplicates within the current contact list. If there are contacts selected within a contacts list then the duplicate detection could operate only on the selected contacts.

-- bye, pabs

https://bonedaddy.net/pabs3/