Support binding a key multiple times

[mathieucaroff]

Current behavior

Currently, when a key is bound multiple times, the last binding operation overrides the previous ones.

Expected behavior

When a key or mouse event is bound multiple times, InputBot dispatches the event to all callback functions, in the order in which they were registered.

Note: this would be a breaking change.

After thinking about it, I found the current API, with KeydbKey:unbind(&self), would make a rather coarse API, requiring to unregister all callbacks at once.

Alternative

Provide a new interface which handles dispatch.

[obv-mikhail]

@mathieucaroff I agree that we would probably want to have a different unbind method. Maybe something like KeydbKey:unbind(&self, fn) where fn is the function bound to the event originally.

How would the alternative with an interface for handling dispatch look like?