Support for naming slicer own threads for debugging.

The following threads are named with this commit:
slic3r_main, slic3r_BackgroundSlicingProcess,
slic3r_tbbpool_xx_yy where xx is a one based index of the TTB thread
and yy is the platform thread ID.

src/slic3r/GUI/Jobs/Job.cpp

@@ -1,6 +1,7 @@
 #include <algorithm>
 
 #include "Job.hpp"
+#include <libslic3r/Thread.hpp>
 #include <boost/log/trivial.hpp>
 
 namespace Slic3r {

src/slic3r/GUI/Jobs/Job.hpp

@@ -3,7 +3,8 @@
 
 #include <atomic>
 
-#include <slic3r/Utils/Thread.hpp>
+#include "libslic3r/libslic3r.h"
+
 #include <slic3r/GUI/I18N.hpp>
 
 #include "ProgressIndicator.hpp"