Use the new drive-harddisk-root icon for the root volume

Summary:
Use the new drive-harddisk-root icon to mark the root volume. This makes it obvious which disk corresponds to / when there are multiple disks, and allows us to remove the root entry from the top section of the Places panel (D15739)

Depends on D16633

Test Plan:

Reviewers: romangg, VDG, Dolphin, Frameworks, bruns, abetts

Reviewed By: VDG, bruns, abetts

Subscribers: abetts, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T8349

Differential Revision: https://phabricator.kde.org/D16653