Fix modal focus trap when the last id changes on the fly

[tesk9]

See Slack for original context.

It seems that having the key event listener using Json.Decode.oneOf to decode events is causing the decoder to not get updated. Investigation steps:

• using focus trap Ellie
• using when focus leaves decoder directly Ellie
• moving the decoder code from when focus leaves into the view Ellie
• removing the Decode.oneOf wrapping from the decoder Ellie

Underlying problem is already issued against elm/json here: elm/json#15

[micahhahn]

Thank you so much for debugging this!