views: Update dt_view_get_image_to_act_on for zoom==1 & full_preview.

Now when zoom == 1 or when full_preview mode is activated, the image to
act on is the only one visible on screen.

This commit adds a lib_only_one_image_seen_id global variable that take
the visible image id when there is only one image visible.
lighttable: Update images_in_row variable.

The lib->images_in_row was only updated when scrolling with a modifier
key activated (always set to 0 in other case). It is needed by the
get_images_in_row function to export this value outside of the lighttable.c
file.

This commit updates lib->images_in_row on filemanager expose and
zoomable table expose.
control/jobs: Update image enumerator when only one image on screen.

Now the libs that are using image enumerator check if there is only one
image displayed on screen. In this case the action is done on that
image.