Rename images with line id

Two main changes:

• Images are renamed {page_id}_{line_number}_{line_id} (previously {page_id}_{line_number}) as it is easier to visualize on Arkindex
• Line number (e.g line_032) should come from the element's name and not from enumerate to avoid offset on pages containing lines without transcription

Note: I'm assuming that there are at most 999 lines/image (line_number has 3 digits)