okular: rewrite snapshot taker to use KIO::PreviewJob

okular: rewrite snapshot taker to use KIO::PreviewJob

KIO::PreviewJob was made for this kind of things, it also does not involve
loading the file (which may play a sound even if the widget is not
visible), seeking, pausing and then grabbing a QWidget snapshot. the only
requirement is for ffmpegthumbs thumbnailer to be installed

Signed-off-by: Ivailo Monev <xakepa10@gmail.com>