Support Double Pages

[OGKevin]

Feature Request: Support Double Pages in ComicReader

References:

• koreader/koreader#13826

Description:
ComicReader should properly support comics where a single image represents two pages stitched together (commonly referred to as a "double page").

Details:

• Detection of double pages could leverage the DoublePage attribute in ComicInfo.xml:

  <xs:attribute default="false" name="DoublePage" type="xs:boolean"/>