Scantailor PDF git20181119

This is an experimental version of Tulon's experimental fork in which he reorganised some stages and implemented multithreading and openCL support. This version adds preliminary support for importing pdf and other image files (JPEG 2000), some ui and other improvements from 4lex4's and trufanov-nok's versions, and general updates to libraries.

Since this is a snapshot, there is no guarantee that anything works correctly and you might very well encounter some bugs or even crashes.