Use different treshold for (auto)scroll in output

Allow, in particular to switch to scolling for longer input (or disable
it) by still keeping the possibility to manually toggle the output to
scroll.

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I ended up doing that because it is **really** annoying to want to see long result coming through and have the output area constantly collapsing, but I still want to be able to collapse if I want to.

With this I can just issue a `IPyton.OutputArea.auto_scroll_treshold = -1` in js console and I'm good to go !