[Folder View] Create KFilePlacesModel only when needed and listen for changes

When no label is displayed, there's no point in creating a KFilePlacesModel.
Also, only emit a change for displayLabel when it actually changed.
Moreover, listen to changes in the KFilePlacesModel and update the label if needed.
Also, make the KFilePlacesModel static and shared between all Folder View instances.

Differential Revision: https://phabricator.kde.org/D14426