Doc: add screenshots for exporting properties

Should have been added as part of f78b6aed2449382e1708688e77d4b0415b8ef6e7

Change-Id: Ic4776b2352ca8e0ea99a341ab8a3397d0eaf3c1d
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>