TextEditors: Keep visible position in editor for 'external' changes

If contents of a text editor change from outside the editor,
the visible portion of the text in the viewport should stay the same.
That is especially apparent when opening a document in a split view, and
adding/removing lines in one of them, above the first visible line in
the other.

Task-number: QTCREATORBUG-11486
Change-Id: I28cde17ecf98cb98c1d6f1259dc66d3671585ee3
Reviewed-by: David Schulz <david.schulz@digia.com>