Remove thing_changed Event traits if possible

[aaronayres35]

Problem Description
I personally find it very confusing to have a trait named thing_changed = Event which explicitly gets set to True when thing changes. Chaco will often listen to that trait when setting up change handlers, but I am not seeing why we couldn't just listen to the trait itself. e.g why do something like observe(_____, thing_changed) instead of just observe(_____, thing).
I may be missing something important as to why this is needed but as far as I can tell it is not.

See #601 which removed the metadata_changed event trait. Other examples include:

chaco/chaco/base_2d_plot.py

Lines 63 to 76 in ef72277

	#: Event fired when the index data changes. Subclasses can listen for this
	#: event and take appropriate steps (except for requesting a redraw, which
	#: is done in this class).
	index_data_changed = Event
	#: Event fired when the index mapper changes. Subclasses can listen for this
	#: event and take appropriate steps (except for requesting a redraw, which
	#: is done in this class).
	index_mapper_changed = Event
	#: Event fired when the value data changes. Subclasses can listen for this
	#: event and take appropriate steps (except for requesting a redraw, which
	#: is done in this class).
	value_data_changed = Event

chaco/chaco/base_2d_plot.py

Lines 278 to 284 in ef72277

	def _update_index_data(self, event=None):
	""" Updates the index data.
	Called by various trait change handlers.
	"""
	self.index_data_changed = True
	self.invalidate_draw()

chaco/chaco/base_2d_plot.py

Lines 313 to 332 in ef72277

	def _index_changed(self, old, new):
	if old is not None:
	old.obseve(self._update_index_data, "data_changed", remove=True)
	if new is not None:
	new.obseve(self._update_index_data, "data_changed")
	self._update_index_data()
	def _value_changed(self, old, new):
	if old is not None:
	old.observe(self._update_value_data, "data_changed", remove=True)
	if new is not None:
	new.observe(self._update_value_data, "data_changed")
	self._update_value_data()
	def _index_mapper_changed(self, old, new):
	if old is not None:
	old.observe(self._update_index_mapper, "updated", remove=True)
	if new is not None:
	new.observe(self._update_index_mapper, "updated")
	self._update_index_mapper()

In any case, if there is a reason that something like this needs to be done, we should still make an effort to clean this code up and make it more easily understandable / better documented of why things are like this.