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