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.