Fix DND onto Task Manager for groups, group dialog scrollbar

Summary:
With this patch drag-hover activation will work for group children,
and moving between different group parents while dragging will
correctly refresh the dialog contents.
BUG:379888

The patch also makes GroupDialog more similar to CompactApplet in
desktoppackage/, in particular managing applet status and activa-
ting the window explicitly. There is no concrete reason for this,
but it seemed like a good idea while debugging the DND issues.

Further, while inverting the MouseHandler<>ScrollArea nesting I
removed some dodgy geometry-setting code for the inside item, and
the ScrollArea viewport now is a single item. This very likely
addresses some reports that the scrollbar sometimes wouldn't appear
when required.
BUG:379037

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6231