[KPhotoAlbum] Doubleclick closes the Viewer

[Christoph Moseler]

Hi Jesper,

another small feature request: In a lot of images viewers it's possible 
to close the full screen window showing an image with a doubleclick.

void Viewer::ImageDisplay::mouseDoubleClickEvent( QMouseEvent* e ) {
     _viewer->close( true );
}

Cheers,
Christoph