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/libslic3r/CMakeLists.txt

@@ -201,6 +201,8 @@ add_library(libslic3r STATIC
     Utils.hpp
     Time.cpp
     Time.hpp
+    Thread.cpp
+    Thread.hpp
     TriangleSelector.cpp
     TriangleSelector.hpp
     MTUtils.hpp