Move some BaseTextDocument setup from BaseTextEditorWidget

There is a 1-to-1 relationship between BaseTextDocument and QTextDocument,
so it doesn't make sense to do the setup between them in the editor widget.

Move creation of BaseTextDocumentLayout and setting up of 'changed' signal
to BaseTextDocument. This also avoids sending multiple
'changed' signals for a single QTextDocument::modificationChanged signal
in case of splits.

Change-Id: Iaab4fea594c5b50f502fd41c88948fac132de9f1
Reviewed-by: Eike Ziller <eike.ziller@digia.com>