Piano: Use CEventLoop::wake() to trigger repaint from sound thread.

In order to repaint the GUI after the sound thread has produced some sweet
new waves, we post a CCustomEvent to the main thread's event loop and then
wake up that event loop via CEventLoop::wake().

Applications/Piano/PianoWidget.cpp

@@ -354,3 +354,11 @@ void PianoWidget::render_knobs(GPainter& painter)
     painter.draw_rect(wave_knob_rect, Color(r, g, b));
     painter.draw_text(wave_knob_rect, wave_name, TextAlignment::Center, Color(r, g, b));
 }
+
+void PianoWidget::event(CEvent& event)
+{
+    if (event.type() == CEvent::Custom) {
+        update();
+    }
+    GWidget::event(event);
+}