[KPropertiesDialog] Show mount information for folders in / (root)

Summary:
Any folder laying in the / partition was excluded to have its 'Mounted on'
and 'Mounted from' displayed in a KPropertiesDialog.

Test Plan:
In dolphin, navigate to Root, right-click on 'etc' to open its properties.
Mounted on and Mounted from should be displayed.

Reviewers: Frameworks, ngraham

Reviewed By: ngraham

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D15945