[Places panel] Don't show Root by default

Summary:
This patch removes Root from the default Places panel for new installations, because it's redundant; you can still get to / on your machine in one click using the appropriate disk entry on the bottom of the Places panel. The appropriate disk entry now has a unique icon to help you distinguish it from others (added in D16653).

By removing it, we gain room to add something more useful such as a Recently Used item (see D7446) without making the Places panel show a vertical scrollbar with Dolphin's default size.

Depends on D16653

Test Plan:
Log in with a new user account or rm ~/.local/share/user-places.xbel and open Dolphin; notice that Root is not on the Places panel.

Existing user accounts are untouched.

Reviewers: Dolphin, VDG, davidedmundson, tcanabrava, ngraham, elvisangelaccio

Reviewed By: Dolphin, VDG, tcanabrava, ngraham, elvisangelaccio

Subscribers: meven, elvisangelaccio, Codezela, davidc, tcanabrava, ndavis, romangg, bruns, davidedmundson, abetts, svenmauch, broulik, acrouthamel, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T8349

Differential Revision: https://phabricator.kde.org/D15739