Set focus to the next current tab when closing the current tab

And by that it means giving the focus to the pageview which is most of
the fimes what we want. One could argue that if i had the focus on the
searchbar we should restore the focus there, but that makes not much
sense to me, since each tab is it's own world, at most one could say,
let's remember where the focus was in that tab the last time it was
focused and restore it there, but it seems a bit convoluted.

To be able of setting the focus to the pageview from the shell we need
to set up some focus proxies, so that part->widget (which is the sidebar)
ends up giving the focus to the pageview, which is what makes sense if
someone says "you part, set yourself the focus"

BUGS: 428257
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