Do not emit iconChanged signal twice if icon url does not change.
This fix implements the same behavior for icon handling in Quick as in
the Widget API.

Change-Id: I9fcc27c9cec3cbef74fb0a71026065e359a5cbc3
Task-number: QTBUG-51179
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>