This includes InputEvent, EventType, EventCategory, and
InputEventCategory

Added the CursorEvent struct to the cursor-related enum variants on
EventType

Also changed how PartialEq and Hash work for EventType. They no longer
consider the inner variant data

These allow us to pass along metadata with a keyboard event
(specifically here, which modifier keys are active)

Re-added event propagation and proper event targeting

Added default_open. Also made it so that open is Optional and determines
the controlled/uncontrolled status of the widget

- Added counter example.
- Added input events and widget event handling.
- Fixed multiple bugs.
- Turned off component diffing until I can figure out a good way of diffing props passed directly to children.