TextEditors: Avoid changing document after construction.

Also when duplicating editors, we don't want to change the document
after construction. Actually at some places (e.g. CppEditorSupport
creation) we don't handle document changes correctly, and we are only
lucky that things still (more or less?) work. Get rid of
BaseTextEditorWidget::duplicateFrom and use copy-constructor style instead.

Change-Id: I7f688b7fcc51d1bb5e222bb333f0d28479b597a6
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>