Fix autosave never save stuff because the image is too busy

Now the strategy is the following:

1. Wait default time period for autosave
2. Check if the image is busy
3. If the image is not busy, just save :)
4. If the image is busy, do three tries with emergency interval (10 ms each). If all the tries are failed, post a background stroke which will run after the current stroke and initiate the autosave.