enable synthetic spreads for all kobos and other devices

[1and2papa]

Kobo readers, e.g. Libra 2, support combing 2 spreads of page when reading in horizontal mode.
Right now, it combines every 2 pages of the generated comic book. That should work in ordinary scenario but in case there are odd pages add between in the books, the Kobo reader might combines the pages in an unexpected way.

I found that there are properties values for itemref that Kobo utilises for identifying the left or right spreads and it will combines the spreads accordingly, which are:

• page-spread-left
• page-spread-right

I observe that KCC does actually identify the spreads and generate left/right spreads by different suffix in idref e.g. kcc, kcc-b or kcc-c, so I'm now fixing this issue by adding such properties with calibre ebook-edit by global replacing the suffix, like below:

From:

<itemref idref="page_Images_kcc-XXX-kcc"/>
<itemref idref="page_Images_kcc-XXX-kcc-b"/>
<itemref idref="page_Images_kcc-XXX-kcc-c"/>

to:

<itemref idref="page_Images_kcc-XXX-kcc" properties="page-spread-left"/>
<itemref idref="page_Images_kcc-XXX-kcc-b" properties="page-spread-right"/>
<itemref idref="page_Images_kcc-XXX-kcc-c" properties="page-spread-left"/>

The above is an example of book in Manga mode(i.e. right to left) and I believe similar mechanism of generating different spreads for left to right is also used in KCC.

My request is simple, it would be nice if KCC can generate such properties in itemref for different spreads. I think there has already been a logic for that so adding such properties should not be a great effort but it improve the reading experience for Kobo owners.