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

The previous patch ebcc7c27edb72c190b3392e254395c9d452f877c changed
the size of the settings struct stored in db making darktable dropping
an old db. Sorry for that...

This patch fixes the same problem (when zoom == 1 or when full_preview
mode is activated, the image to act on is the only one visible on
screen) using lighttable.c dt_library_t struct instead of using settings
struct stored in db.

A proxy was added to view_manager to be able to get the
one_image_seen_id from the dt_view_get_image_to_act_on function.