Don't reset model when activeconnectionstate changed

Summary: resetting the model when a single property changes is using a sledgehammer to crack a nut. emitting dataChanged for the affected row is enough.

Test Plan:
(de)activated connections in plasmoid and mobile wifi KCM.
Verified with hotspot that lots of calls to modelReset disappeared

Reviewers: jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24903