Find elements recursively even when there are no non-folder direct children

Closes #3 (closed)

When a folder only has folder elements as its direct children, get_first_images assumed there were no children in this folder at all, so it did not try to fetch them recursively.

changed milestone to %Arkindex 1.6.3

requested review from @babadie

assigned to @erouchet

merged

mentioned in commit 2afe05d6