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 {nav root} entry from the top section of the {nav Places} panel (D15739)

Depends on D16633

Test Plan:
{F6388890}

{F6388889}

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