Don't keep a full NetworkModel instance alive when it's not visible

Summary:
This was implemented as an optimisation in Plasma at some point, where all full
representations are displayed at all times.
This would mean doing synchronous calls at start querying every wifi connection
we ever did and keeping the view fresh at all times.
This patch changes it so the networks model is only present when the full
representation is on display.

Test Plan: Tested by hand both with plasmoidviewer and a full plasmashell.

Reviewers: #plasma, jgrulich

Reviewed By: jgrulich

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21501