Thumbnails for projects need to be updated

Thumbnails for projects need to be updated.

When the project is initially opened we generate a thumbnail. 
But we do not automatically regenerate a thumbnail when the project is changed.

App.qml is usually not changed updated, but other files that influence how App.qml is looking do change.

When a new project is generated the thumbnail is usually a gray rectangle. 

Regenerating the thumbnail on close has issues, but we could remove outdated previews from the cache so that they get regenerated.