In the library, this option will autoload the images in the newly opened
directory.

Related to #694 and #132.

Instead of having the working directory handler directly load the
directory, we now defer this to the LibraryModel. This way, the model
can decide if the path selected in the new directory is an image. If
yes, load it - with this image focused.

Thus, we don't end up jumping positions like previously, when we enter a
directory that:
* Has images
* Has an image in the stored position
Where:
1) We would load the directory into the library => paths visible in
   library
2) We would load the stored position => new path selected
3) We would load all images into the image filelist
4) We would open the first image
5) The library would select the first image => jump to new path which is
   not desirable

1) The stored path is an image => load images and select this one
2) There are images in the directory => load images and select the first
   image, overriding any stored directory
3) There are no images in the new directory => clear images

This comes very close to the preferred behaviour in #132, except for the
async part which we are omitting, in favour of the *optional*
--open-selected flag.

This emulates the behaviour of :scroll left/right --open-selected by
directly setting autoload in the library.

The image filelist should be empty when running
:scroll left --open-selected and entering a directory without any
images.

React to the emitted signal, instead of exclusively emitting it.