cre: adds isLinkToFootnote() and highlightXPointer() (#741)

Footnote detection code is quite huge, and tweakable from
Lua with the use of flags.

Also adds optional parameter to getPageLinks() to request only
internal links.
Also have getPageLinks() add to each link the segments (rect for
each line) it spans on the page, for a more accurate distance
from gesture position computation.