Fix image loading performance

Summary: Start the dirlister after loading an image whose url is passed on the command line to make sure that this image is shown before the possibly long running dirlister on a potentially slow device is started.

Test Plan:
Mount some slow network filesystem (e.g. cifs) and open an image file
from a directory containing a few hundred files using the command line:
$ gwenview imagefile

It will take several seconds/minutes until the image is shown (until all
preview images are loaded).

After applying this patch the image will be shown immediately and the
preview images after that.

Reviewers: #gwenview, ngraham

Reviewed By: #gwenview, ngraham

Subscribers: ngraham, tommo

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D25005