applet: Fix network filtering

Using fixed text and inherited filtering logic makes more sense than
trying to re-implement it in subclass.

Regarding EditorProxyModel which is used in KCM code, Qt::DisplayRole
turns out to be always empty, as no source model in the chain populates
this role with any data. Thus, it should be safe to switch to the
custom NetworkModel::NameRole completely.

This is forward compatible with Qt 6, and backward compatible with up
to 5.12 versions.