WatcherImpl may have a detach method

[PackRuble]

I see this as introducing two possible methods:

• detach(Card) - will remove all listeners for this card
• detachAll() - will remove all listeners for all cards altogether

At the moment, there is no way to implement the removal of a specific callback tied to a specific card. The opinion is based on the technical implementation of WatcherImpl.

For what?
It might be worth considering some use cases:

• you don't use detacher (perhaps because of its implementation, which is not convenient for all cases) in the attach method and want to delete [all] callbacks yourself later
• you don't need to remove callbacks. Your application adds them once and uses them after until you close the application.

This would also require making detacher a nulable-type (but probably still required)