Add QQuickTextEdit::append()

This makes it possible to append text more efficiently than
appending to the text -property, and also avoids weird rich
text formatting issues with the latter approach.

Task-number: QTBUG-31575
Change-Id: Id621773588b94e36f8f0b9eb6b22590e9db62811
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>