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.
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