Optimise the NetworkModel

Summary:
Only notify changes about the bits that actually changed.
This is especially important for sorted lists, as a small change on a detail could trigger a resort and redraw of the whole view, especially at boot.

Reviewers: Plasma, jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21307