(re)add mechanism for a callback in any glib event loop executed before the loop "executes"

This is based on code from earlier commits that were later reversed, but we need some mechanism
to ensure that threads have a thread local tempo map ptr set. The big difference is that this
time we do not implement this for all instances of an AbstractUI - implementation is left to
each thread/event loop

libs/pbd/base_ui.cc

@@ -175,5 +175,6 @@ BaseUI::attach_request_source ()
 {
 	DEBUG_TRACE (DEBUG::EventLoop, string_compose ("%1: attach request source\n", event_loop_name()));
 	request_channel.attach (m_context);
+	maybe_install_precall_handler (m_context);
 }
 

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;
+}

libs/pbd/pbd/base_ui.h

@@ -105,6 +105,8 @@ class LIBPBD_API BaseUI : public sigc::trackable, public PBD::EventLoop
 	void signal_new_request ();
 	void attach_request_source ();
 
+	virtual void maybe_install_precall_handler (Glib::RefPtr<Glib::MainContext>) {}
+
 	/** Derived UI objects must implement this method,
 	 * which will be called whenever there are requests
 	 * to be dealt with.

libs/pbd/pbd/glib_event_source.h

@@ -0,0 +1,34 @@
+#ifndef __libpbd_glib_event_source_h__
+#define __libpbd_glib_event_source_h__
+
+#include <boost/function.hpp>
+
+#include <glibmm/main.h>
+#include "pbd/libpbd_visibility.h"
+
+class LIBPBD_API GlibEventLoopSource : public Glib::Source
+{
+  public:
+	GlibEventLoopSource () {};
+
+	bool prepare (int& timeout);
+	bool check();
+	bool dispatch (sigc::slot_base*);
+};
+
+
+class LIBPBD_API GlibEventLoopCallback : public GlibEventLoopSource
+{
+  public:
+	GlibEventLoopCallback (boost::function<void()> callback) : _callback (callback) {}
+
+	bool check() {
+		_callback();
+		return false;
+	}
+
+  private:
+	boost::function<void()> _callback;
+};
+
+#endif /* __libpbd_glib_event_source_h__ */

libs/pbd/wscript

@@ -51,6 +51,7 @@ libpbd_sources = [
     'file_archive.cc',
     'file_utils.cc',
     'fpu.cc',
+    'glib_event_source.cc',
     'id.cc',
     'locale_guard.cc',
     'localtime_r.cc',