[Folder View] Remember selected item when navigating in subfolders

Summary:
When using the folder view in a panel as a popup, being in list view
mode, getting into subfolders and back with arrow keys had a weird
behaviour: the currentItem would just stay the same accross folders.

This patch restores both the proper selected item and the scrolling
position when navigating back.

BUG: 359310

Test Plan:

• Add a folder view in a panel, configure it in list view mode
• Enter a subfolder and go back

The folder you had gone into should be selected and the scrolling
position should be the same than when you entered it.

Reviewers: Plasma, broulik, hein

Reviewed By: Plasma, hein

Subscribers: anthonyfieroni, ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15840