Add indicator dots to spine of outer folio of each signature

[sithel]

see: https://discord.com/channels/726108551428440206/953709370267541634/1167723417529753720 (and then down a bit the conversation resumes/continues)

The idea is that the outside of the outermost folio gets dots at key points along the spine (for help with trimming and/or punching)

This is particularly important on layouts that have buffer space at header and footer that one might want to trim w/o risking trimming of text. Would pair well with the pending #55 if creators shifted their margins from the PDF into the Imposer tool

Shouldn't take a lot of work to add, just need to figure out where to best surgically insert it into the code... (and what exactly is the enabling/configuring UI?)

please ping badgertide if this gets done

[acestronautical]

The UI is tricky. I guess the elements could be:

• number of tapes to sew on
• width of tapes to sew on
• margin/offset from page edge to start markings.

People not sewing on any tapes could still select IE 1 tape and 2 inches wide tape or something, and people sewing on cords could select tapes with little to no width. I think this UI would address the most common use cases. Default might be 3 tapes, 1/2 inch wide, and start markings 3/4 in from the edge? This layout would look like:

|                                                                                         |
-------------------------------------------------------------------------------------------
|<3/4inch>○<Spacing>○<1/2inch>○<Spacing>○<1/2inch>○<Spacing>○<1/2inch>○<Spacing>○<3/4inch>|
-------------------------------------------------------------------------------------------
|                                                                                         |

Where spacing would be = (page height - (2 * margin + numTapes * tapeWidth)) / (numTapes + 1). Maybe my math is wrong, but basically free space over number of gaps between tapes. If this were an 8 1/2 height page I believe the spacing would be 1 3/8.
I think you can assume that all layouts will be symmetrical and evenly spaced.

This gets trickier if you want to support weird hole layouts https://ariellesbindery.com/2019/10/20/models-of-bookbinding-techniques-sewing-structures/

Could potentially just let the user enter the raw locations of each mark they want, but that feels bad.

Also this says outer folio, but might want to support choosing outside of outer folio or inside of inner folio. It's hard to punch an awl accurately through the backside of a folio.