Overhaul group popup dialog

Summary:
A set of UI improvements to the group popup dialog:
* The dialog was height-limited but not scrollable. It now shows
  a vertical scrollbar if needed.
* The existing keyboard handling is extended to scroll any item
  navigated to by keyboard into view.
* At opening time (before the dialog causes a change in window
  focus) the currently-active task (if any) is collected, and
  then made the active item, scrolling it into view if needed.
  Keyboard nav will then start there. This improves initial
  focus handling.
* The normal wheel-handling is short-circuited in favor of
  scrolling when the dialog overflows.

BUG:375196

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4229