Focus handling fixes

Summary:

• Fix keyboard nav on the desktop again.

BUG:399566

• Fix initial focus properly.
• Make sure when using arrows after starting Plasma or after clicking into empty space, the first selected item is always 0, not 1 or some random item based on previous activity.
• Make sure the view is always scrolled up when (re)opening the popup.

Reviewers: broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D16106