[KPhotoAlbum] Viewer startup performance problem

[Robert L Krawitz]

All of the MainWindow::Window viewer launch methods call
selectedOnDisk(), which goes through each selected image and
determines whether it's on disk.  This causes a big performance
degradation if the number of images being viewed is large and their
inodes aren't in cache; the experience is that the viewer takes many
seconds to start up.

I tried removing the imageOnDisk() check, and on a superficial test
nothing really bad happened; images not on disk simply appeared
blank.  Since the normal case is that images will always be on disk,
perhaps that check could be removed?