Thread names shortened to 15 characters to fit Posix norm.

Added get_current_thread_name()

src/slic3r/GUI/BackgroundSlicingProcess.cpp

@@ -224,7 +224,7 @@ void BackgroundSlicingProcess::process_sla()
 
 void BackgroundSlicingProcess::thread_proc()
 {
-    set_current_thread_name("slic3r_BackgroundSlicingProcess");
+    set_current_thread_name("slic3r_BgSlcPcs");
 	name_tbb_thread_pool_threads();
 
 	assert(m_print != nullptr);