[plasma-nm/applet] Add right-click context menu to directly customize a connection

Summary: {F6566071}

Reviewers: #vdg, jgrulich

Reviewed By: jgrulich

Subscribers: broulik, jgrulich, ngraham, abetts, GB_2, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18504