Implement a replacement for Glib::Threads::Thread

This is mainly because Glib::Threads (g_system_thread_new)
uses pthread on Un*x, but on Windows relies on GThreadWin32
(HANDLE)_beginthreadex

This later causes issues e.g. in BaseUI::run()

```
unhandled exception (type Glib::Error) in signal handler:
domain: g_thread_error
code  : 0
what  : Error setting new thread priority: The parameter is incorrect.
```

libs/pbd/pbd/pthread_utils.h

@@ -93,6 +93,26 @@ LIBPBD_API bool pbd_mach_set_realtime_policy (pthread_t thread_id, double period
 namespace PBD {
 	LIBPBD_API extern void notify_event_loops_about_thread_creation (pthread_t, const std::string&, int requests = 256);
 	LIBPBD_API extern PBD::Signal3<void,pthread_t,std::string,uint32_t> ThreadCreatedWithRequestSize;
+
+	class LIBPBD_API Thread {
+		public:
+		static Thread* create (boost::function<void ()> const&, std::string const& name = "");
+		static Thread* self ();
+		void join ();
+		bool caller_is_self () const;
+
+		private:
+		Thread ();
+		Thread (boost::function<void ()> const&, std::string const& name = "");
+		Thread (Thread const&); /* precent copy-construction */
+
+		static void* _run (void*);
+
+		pthread_t                _t;
+		std::string              _name;
+		boost::function<void ()> _slot;
+		bool                     _joinable;
+	};
 }
 
 /* pthread-w32 does not support realtime scheduling

libs/pbd/pthread_utils.cc

@@ -28,6 +28,7 @@
 #include <dlfcn.h>
 #endif
 
+#include "pbd/failed_constructor.h"
 #include "pbd/pthread_utils.h"
 
 #ifdef COMPILER_MSVC
@@ -436,3 +437,65 @@ pbd_mach_set_realtime_policy (pthread_t thread_id, double period_ns, bool main)
 #endif
 	return false; // OK
 }
+
+PBD::Thread*
+PBD::Thread::create (boost::function<void ()> const& slot, std::string const& name)
+{
+	try {
+		return new PBD::Thread (slot, name);
+	} catch (...) {
+		return 0;
+	}
+}
+
+PBD::Thread*
+PBD::Thread::self ()
+{
+	return new PBD::Thread ();
+}
+
+PBD::Thread::Thread ()
+	: _name ("Main")
+	, _joinable (false)
+{
+	_t = pthread_self ();
+}
+
+PBD::Thread::Thread (boost::function<void ()> const& slot, std::string const& name)
+	: _name (name)
+	, _slot (slot)
+	, _joinable (true)
+{
+	pthread_attr_t thread_attributes;
+	pthread_attr_init (&thread_attributes);
+
+	if (pthread_create (&_t, &thread_attributes, _run, this)) {
+		throw failed_constructor ();
+	}
+}
+
+void*
+PBD::Thread::_run (void* arg) {
+	PBD::Thread* self = static_cast<PBD::Thread *>(arg);
+	if (!self->_name.empty ()) {
+		pthread_set_name (self->_name.c_str ());
+	}
+	self->_slot ();
+
+	pthread_exit (0);
+	return 0;
+}
+
+void
+PBD::Thread::join ()
+{
+	if (_joinable) {
+		pthread_join (_t, NULL);
+	}
+}
+
+bool
+PBD::Thread::caller_is_self () const
+{
+	return pthread_equal (_t, pthread_self ()) != 0;
+}