FIX: fix the crash issue #1944 caused by wronog thread name in some

corner cases

this patch is cherry-picked from PrusaSlicer's 84722876012ca310e29b291e10fef9d18ae26cea
thanks to 'Vojtech Bubnik' for the fix

Implementing a test whether the current thread is the main (UI) thread
and using it on AppConfig::save() to assert if save is called
from a worker thread.
The old assert was using thread names, which did not always work
on Windows.
Fixes #7839 #9178 #9370 #9420

src/libslic3r/Thread.cpp

@@ -187,6 +187,26 @@ std::optional<std::string> get_current_thread_name()
 
 #endif // _WIN32
 
+// To be called at the start of the application to save the current thread ID as the main (UI) thread ID.
+static boost::thread::id g_main_thread_id;
+
+void save_main_thread_id()
+{
+	g_main_thread_id = boost::this_thread::get_id();
+}
+
+// Retrieve the cached main (UI) thread ID.
+boost::thread::id get_main_thread_id()
+{
+	return g_main_thread_id;
+}
+
+// Checks whether the main (UI) thread is active.
+bool is_main_thread_active()
+{
+	return get_main_thread_id() == boost::this_thread::get_id();
+}
+
 // Spawn (n - 1) worker threads on Intel TBB thread pool and name them by an index and a system thread ID.
 // Also it sets locale of the worker threads to "C" for the G-code generator to produce "." as a decimal separator.
 void name_tbb_thread_pool_threads_set_locale()