[Feature] Check or un-check the Flags based on hotkeys

[AnakTeka]

So I'm trying to label multi class multi label classification, I found out the existence of Flags helped me. But, it would be really great if we could able to check or un-check the Flags based on hotkey, as this would hugely speed up the labeling process. I have tried to do some workaround around the source code to add additional hotkey but couldn't figure it out.