Use Kio::KPlacesModel as source model for PlacesItemModel

Summary:
Use Kio::KPlacesModel as source model for PlacesItemModel avoiding
duplicated code.

Depends on D8862
Depends on D8332
Depends on D8434
Depends on D8348
Depends on D8630

Test Plan: Unit test created

Reviewers: elvisangelaccio, emmanuelp, mlaurent, mwolff

Reviewed By: elvisangelaccio, mlaurent, mwolff

Subscribers: mwolff, mlaurent, anthonyfieroni, nicolasfella, ngraham, #dolphin

Differential Revision: https://phabricator.kde.org/D8855