Fix PlacesItemModel loading with hidden devices

Summary:
Make sure that hidden devices loaded after module initialization does
not appear in the view.
Check for item visibility before add it on the model.

Test Plan: Unit test created

Reviewers: franckarrecot

Reviewed By: franckarrecot

Subscribers: cfeck, ervin, Dolphin

Differential Revision: https://phabricator.kde.org/D9441