Invisible part setting field.
L18n compliance for pressbooks-book cover page, other l18n fixes.
Colon after copyright in webbook footer.
Initial implementation of invisible parts.
Merge branch 'invisible' into dev
Prince styles for invisible parts.
Part that have content are new linked in TOC on webbooks.
Added 5" x 8" page size for PDFs.
SVG files are now enabled.
Merge branch 'dev' into epub3