fix import-thread SNAFU

* pressing Esc or WM close button did not cancel import thread
 * proper Abort does not wait for import thread either

It was possible to launch a 2nd (and Nth) import thread, all sharing
the Editor's ImportStatus data-structure, all having the same
registered thread-name and same thread-pool name.  Plenty of room for
crashes.

gtk2_ardour/editor_audio_import.cc

@@ -507,7 +507,10 @@ Editor::import_sndfiles (vector<string>            paths,
 		gtk_main_iteration ();
 	}
 
-	import_status.done = true;
+	// wait for thread to terminate
+	while (!import_status.done) {
+		gtk_main_iteration ();
+	}
 
 	int result = -1;
 

gtk2_ardour/interthread_progress_window.cc

@@ -55,6 +55,14 @@ InterthreadProgressWindow::InterthreadProgressWindow (ARDOUR::InterThreadInfo* i
 	Glib::signal_timeout().connect (sigc::mem_fun (*this, &InterthreadProgressWindow::update), 100);
 }
 
+void
+InterthreadProgressWindow::on_hide ()
+{
+	if (!_interthread_info->done) {
+		_interthread_info->cancel = true;
+	}
+}
+
 void
 InterthreadProgressWindow::cancel_clicked ()
 {

gtk2_ardour/interthread_progress_window.h

@@ -42,6 +42,7 @@ public:
 protected:
 
 	virtual bool update ();
+	virtual void on_hide ();
 
 	Gtk::Button _cancel_button;
 	Gtk::Label _cancel_label;