domEvent should support key

[allevo]

Currently, SeqFlow forces us to use { el: button } to attach an event handler to a HTMLElement. While this is technically correct, it is not developer-friendly.

SeqFlow already uses key for other purposes; we can use { key: 'incrementButton' } to specify the element you want to attach to the handler.

NB: it seems easy to implement, but it could be tricky when you use the domEvent method after replaceChild on the same element. What should happen in that case?