[Snowcap] Let captured input go through

[Ph4ntomas]

Snowcap have an internal subscription on keyboard events, but the events are filtered on the server side.

I would like the ability to handle some key events from the config, regardless of if a widget 'captured' it (the 'captured' flag could be sent so the config can the decide how to process the event.

For context:
I'm using a TextInput for my prompt (from #369).
The default behavior is that the TextInput captures keyboard inputs while focused.

I'd like to add the following behaviors:

• Close the prompt on Escape => The first Escape is captured by the TextInput, which unfocus itself. The config is not notified of this change.
• Close the prompt on Backspace if the field is empty => The BackSpace is always captured by the TextInput.