[glib] doc: Move PopplerColor documentation to it's own section

https://bugs.freedesktop.org/show_bug.cgi?id=79798
[glib] doc: Move poppler_get_{version, backend} documentation to features section

https://bugs.freedesktop.org/show_bug.cgi?id=79798
[glib] doc: Move PopplerPrintFlags to PopplerPage section

https://bugs.freedesktop.org/show_bug.cgi?id=79798
[glib] doc: Move PopplerFindFlags to PopplerPage section

And also add the missing POPPLER_TYPE_FIND_FLAGS and poppler_find_flags_get_type.

https://bugs.freedesktop.org/show_bug.cgi?id=79798
[glib] doc: Add poppler_annot_set_flags() to documentation

https://bugs.freedesktop.org/show_bug.cgi?id=79798
[glib] doc: Fix typo

https://bugs.freedesktop.org/show_bug.cgi?id=79798
[glib] doc: Add missing POPPLER_TYPE_POINT

And while we are here, also move poppler_point_get_type() and
poppler_quadrilateral_get_type() to private subsection.

https://bugs.freedesktop.org/show_bug.cgi?id=79798
[glib] doc: Add PopplerBackend documentation

https://bugs.freedesktop.org/show_bug.cgi?id=79798
[glib] doc: Use <emphasis> instead of <em>

Fixes:
  DOC   Building HTML
Element em in namespace '' encountered in para, but no template matches.

https://bugs.freedesktop.org/show_bug.cgi?id=79798
[glib] doc: Document PopplerFindFlags:POPPLER_FIND_DEFAULT

Fixes warning:
  DOC   Building XML
../../glib/poppler.h:157: warning: Value description for
PopplerFindFlags::POPPLER_FIND_DEFAULT is missing in source code comment
block.

https://bugs.freedesktop.org/show_bug.cgi?id=79798
[glib] doc: Move poppler_date_parse to utility functions section

https://bugs.freedesktop.org/show_bug.cgi?id=79798