add new GSource-derived object that can be attached to a Glib::MainContext to execute code before other sources have their dispatch() method invoked

libs/pbd/glib_event_source.cc

@@ -0,0 +1,19 @@
+#include "pbd/glib_event_source.h"
+
+bool
+GlibEventLoopSource::prepare (int& timeout)
+{
+	return false;
+}
+
+bool
+GlibEventLoopSource::check ()
+{
+	return false;
+}
+
+bool
+GlibEventLoopSource::dispatch (sigc::slot_base*)
+{
+	return false;
+}