Make adding symbols to the diagram possible.

This required creating a simple framework for operations to LdCanvas.
It is probable that it's going to require some changes when other
operations are added. The basic idea behind it seems to be good, though.

src/ld-window-main.c

@@ -761,7 +761,25 @@ on_canvas_button_release (GtkWidget *widget, GdkEventButton *event,
 	if (event->button != 1)
 		return FALSE;
 
-	/* TODO: Add the selected symbol into the document on the position. */
+	if (data->active_item != -1)
+	{
+		LdDiagramSymbol *symbol;
+		const gchar *category_name, *symbol_name;
+		gchar *klass;
+
+		category_name = ld_symbol_category_get_name
+			(g_object_get_data (G_OBJECT (data->active_button), "category"));
+		symbol_name = ld_symbol_get_name
+			(data->items[data->active_item].symbol);
+
+		klass = g_build_path (LD_LIBRARY_IDENTIFIER_SEPARATOR,
+			category_name, symbol_name, NULL);
+		symbol = ld_diagram_symbol_new (klass);
+		g_free (klass);
+
+		ld_canvas_add_object_begin (self->priv->canvas,
+			LD_DIAGRAM_OBJECT (symbol));
+	}
 
 	/* We've either chosen a symbol or canceled the menu, so hide it. */
 	if (data->active_button)