Add sorting hook for dw2pdf plugin

[annda]

Recent refactoring of dw2pdf has introduced a new event DW2PDF_NAMESPACEEXPORT_SORT.

This PR adds a simple sort function which sorts pages based on their indexmenu tag (indexmenu_n).

This sorting will only take place if dw2pdf receives parameter book_order=indexmenu or book_order=indexmenu_strict. In the strict mode, tags are required for PDF export and an exception will be thrown if one of the pages is missing a tag.