Added ComponentScope.stateIO
SyntheticEvent now covariant
SyntheticEvent.dragEvent added
Added {preventDefault,stopPropagation}{F,IO}
Added component state modStateIOM, tweaked runStateIO*
Added ComponentStateFocus
Revised how optional callbacks are handled
modState() now only stores the new state if it's changed.
HTML5 attributes for drag-and-drop.
Added modState{O,U}
Merge branch 'master' into underlying_constructor_access

* master:
  Added modState{O,U}
  HTML5 attributes for drag-and-drop.
  modState() now only stores the new state if it's changed.
  Revised how optional callbacks are handled
  Added ComponentStateFocus
  Added component state modStateIOM, tweaked runStateIO*
  Added {preventDefault,stopPropagation}{F,IO}
  SyntheticEvent.dragEvent added
  SyntheticEvent now covariant
  Added ComponentScope.stateIO