Allow sorting pages using indexmenu

[ZbigniewTomanek]

This modification solves the problem faced by users who want to print a pdf using their own page order.

A new sorting mode indexmenu has been added, which uses the {{indexmenu_n>}} tag used by the indexmenu plugin. In addition, the indexmenu_mode parameter has been added, taking values of

• strict - will raise an error if {{indexmenu_n>}} tag is not present in any of printed pages
• soft - pages without {{indexmenu_n>}} tag will be sorted alphabetically

Example print url: http://localhost/wiki:dokuwikit?do=export_pdfns&book_ns=wiki&mode=strict&book_title=Wiki&book_order=indexmenu&toclevels=1-3

[splitbrain]

I'd prefer to have the dw2pdf plugin use an event that allows other plugins to extend/modify the collection of pages to be added. Then this could be added to the indexmenu plugin or implemented as a third plugin.

In general the dw2pdf Plugin would need a bit of cleanup and refactoring to make that easily possible.

The code here also has some problems in the details (like the use of a global) but it doesn't make sense to review this in detail, when at least I would be reluctant to merge this at all.