Bug 594153 (3/3) - Allow setting alarms on any meeting

By tying the attachment view "editable" action group to comp-editor
"individual" group, these add/remove actions are marked sensitive /
insensitive in sync with the existing "Insert" menu items which are
already in the "individual" group.

Though not introduced with the other symptoms of this bug
(0597b877), the descreptency fixed
here is integrally related.