Designer: Remove some redirections.

Instead of keeping a separate plain text editor and document,
the FormWindowEditor and FormWindowFile are directly derived
from PlainTextEditor and BaseTextDocument respectively.

Change-Id: I4319904dea769ec31900061bc7c3a3c5c22e0e8a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>