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/task-manager.c

@@ -266,6 +266,7 @@ model_update_tree_iter (GtkTreeModel *model, GtkTreeIter *iter, Task *task)
 
 	g_free (background);
 	g_free (foreground);
+	g_free (old_state);
 }
 
 static void