Add a length property to TextEdit.

This returns the length of the unformatted text in a TextEdit which
should be cheaper to query than the length of the text property and
meaningful in the context of the selection and cursor properties.

Task-number: QTBUG-18949
Change-Id: Ia25c4553693923f97d299f1fdb8bfcf7f5937b13
Reviewed-by: Martin Jones <martin.jones@nokia.com>