Add scroll_visible method for focused children

[willmcgugan]

We need a scroll_visible (or better name) method that scrolls a widget in to view.

This should be called when a widget receives focus so that tab-to-focus doesn't something that is not currently visible.

There should be an animate on the method so we can chose to animate it or note.

[olivierphi]

Would we automatically handle this recursively, so that all the Widget's parents (not only its 1st level parent) scroll too if necessary?

[willmcgugan]

https://github.com/Textualize/textual/wiki/Sorry-we-closed-your-issue

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