EVENT: Use a query selector for the element spec

[pauleveritt]

In the proposal, you might write: @handler( event="mousemove", element=Element("empty_button").element )

Instead, I propose allowing element: Element | str. If it is a string, then use document.querySelector. This would have a few benefits:

• It's nicer. 😀
• It doesn't require evaluation at import time.
• It could match multiple nodes and attach handlers to them.

[pauleveritt]

On the last point...we'd want to make sure we don't register a pile of proxies for every matching node. For example, a Delete button on a row of a todo app. (Of course, people should register the handler on the container, but...)