Fix connections' context menus position and parent

Summary:
A connection's context menu now opens only from the main item, not from the expanded details;
this allows again copying the connection information (as per comments in D18504).

Reviewers: broulik, jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18745