fix and simplify the poppler search and usage

- drop the (braindead) FindPoppler.cmake module, just use the much simplier PopplerQt4 provided with kdelibs
(since only poppler-qt4 is used, all the stuff for xpdf headers is unneeded)
- use the right variables for include dirs (now taken into account) and libraries
- fix the #include's, <poppler-qt4.h> is enough when the poppler include dirs are used
- remove one uneeded poppler-qt4.h #include