Transmit DisplayFrameSync message, handle in WaterfallSpectrum.

More kludginess, especially around initialization and timing. But it addresses the flickering lines of pixels at the bottom of the waterfall scroll area!

firmware/application/main.cpp

@@ -197,6 +197,8 @@ private:
 	}
 
 	void handle_lcd_frame_sync() {
+		DisplayFrameSyncMessage message;
+		context.message_map().send(&message);
 		painter.paint_widget_tree(top_widget);
 	}