Respond appropriately to textFormat changed

If the text has not changed when textFormat changes between states other
than RichText, the text was not re-laid-out. This is necessary if the
text includes control tags.

Task-number: QTBUG-31191
Change-Id: I3c6f5343aa85e8337b90cf86748a696d5742e906
Reviewed-by: Andrew den Exter <andrew.den.exter@qinetic.com.au>