Implement click events

[JoshKarpel]

https://stackoverflow.com/questions/14805225/whats-the-difference-between-mouseup-and-click-events

[JoshKarpel]

#112 means you can sort of implement click events in user space with a use_state that tracks whether the element was mouse-downed, and a handler for both mouse down and up events. But it's not clear how you'd clear the click state if the button is released over another element - that might be the main reason to push this into the framework.

Typical browser behavior is that it's a click if-and-only-if the mouse down and corresponding up happen on the same element - but it doesn't matter whether the mouse moves across any other elements between those events.