network-flow: let a widget open a panel about itself, and give focus back

The two detail panels are keyed by an entity - an address, a number - and
that is all their openers carry. A breakdown of a donut's remainder cannot
be asked for that way: which categories are behind the slice depends on the
widget's limit, and whether the answer matches what the reader is looking at
depends on its filters and window. So this opener takes the widget's own
configuration and context, and the identity check that decides whether to
remount becomes unconditional - two targets are objects, and a drill-down
should re-ask its question anyway.

Closing now returns the keyboard to whatever opened the panel. The panel
prevents the dialog library's own restoration and is unmounted on the way
out, so focus was landing on the document body: a reader who pressed Enter
on a legend row and then Escape was put back at the top of the dashboard.
The trigger is remembered when the panel opens - an arc counts, the donut's
slices are focusable groups - and refocused when it closes, which covers
Escape, the close button and the overlay alike. The two existing panels get
that for free.

CMK-37878

Change-Id: Ie58476090c65c2ada8ca8ac92fe79b87b62d487f