TextEditors: Fix document changed indicator in case of splits

Since the changed signal was moved from the editor to the document, we
now need to set up all the connections in the setupDocumentSignals.
Otherwise split editors, which get a different document after creation,
do no longer set up the right connections.

Task-number: QTCREATORBUG-10301
Change-Id: Ifc7b426abf87ab73ea37379c1a55e5af2005fc43
Reviewed-by: David Schulz <david.schulz@digia.com>