Add UI code to open gpgolkeyadder from explorer

* src/gpgoladdin.cpp (GetCustomUI_MIME): Add code for
contacts view in explorer.

This is the main commit that adds our address book functionality
to the explorer, too. This is important because in recent
Versions of Outlook the "Full On" Contact editor is no longer
very prominent so that Outlook users might never notice our
Addressbook functionality.

• GnuPG-Bug-Id: T4874