Use childScope in sentenceList

Very dangerous. It was reusing its parent scope before - others do this
as well. We had a variable clash here, where the badly named n for
navigator was overridde with n as in notifier...
HACK - make main-body unscrollable

The tree canvas keeps overflowing the main-body and adds an unnecessary
scrollbar to it.
Generally we want scroll functionality in the main-body so we will have
to investigate this further. In a treebanking only environment we won't
have a need for such a scrollbar though - for now we can just disable
scrollability safely and no one will notice...