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