Plug a dynamic leak

A leak occured in the timer code, and thus the memory was growing over
time. Cf. src/task-manager.c(model_update_tree_iter), the old_state
variable was not free'd.

Plugged other static leaks at the same time.

src/main.c

@@ -140,6 +140,9 @@ int main (int argc, char *argv[])
 	if (timeout > 0)
 		g_source_remove (timeout);
 	g_object_unref (window);
+	g_object_unref (status_icon);
+	g_object_unref (task_manager);
+	g_object_unref (settings);
 
 	return 0;
 }