core:EventDispatcher. Intruduce `bound_events` method that returns a
dict of properties and events of the Widget along with a list of methods
that are bound to the property/event.
core:events rename bound_events to `get_property_observers` and return
only the observers for the property name passed.
core:Events `events` method for returning a list of events of a widget
Merge pull request #1310 from kivy/_events

core:EventDispatcher.