Peter M. Groen / openbr

Release a thread from the global thread pool whenever we wait for a parallel
job to finish. This means we don't waste one of the global thread pool's
threads, and can maintain full CPU utilization.

Add Show2Transform (should replace ShowTransform eventually), which displays
input images using QT, rather than going through openCV.
Additional modifications to support doing this (since qt is quite particular
about how GUI elements are created/modified). Create a QApplication instead
of a QCoreApplication (if not building BR_EMBEDDED), restructure br.cpp's main
to run its main work in a separate thread while the main thread sits in an
event loop.