photosurface demo: behavior depends on initialization order

The file dialog wasn't appearing on the mac.  This way seems to work.
We have a long-term bug here but this workaround will look better for
the release.

Change-Id: I8538ca5dc11a063591bc4557fd3779c5bb3768b5
Task-number: QTBUG-31699
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>