PreferencesHelper does not distinguish defined trait named "*_items" from _items event trait for listeners

[kitchoi]

In PreferencesPage, we have this block of code that synchronizes changes on the PreferencesPage with the underlying Preferences:

apptools/apptools/preferences/ui/preferences_page.py

Lines 66 to 85 in d755bf1

	def _anytrait_changed(self, trait_name, old, new):
	"""Static trait change handler.
	This is an important override! In the base-class when a trait is
	changed the preferences node is updated too. Here, we stop that from
	happening and just make a note of what changes have been made. The
	preferences node gets updated when the 'apply' method is called.
	"""
	# If the trait was a list or dict '_items' trait then just treat it as
	# if the entire list or dict was changed.
	if trait_name.endswith("_items"):
	trait_name = trait_name[:-6]
	if self._is_preference_trait(trait_name):
	self._changed[trait_name] = getattr(self, trait_name)
	elif self._is_preference_trait(trait_name):
	self._changed[trait_name] = new

#196 makes the PreferencesHelper (the base class of PreferencesPage) consistent in the handling of _items trait: It is assumed to be the event trait for listening to list/dict/set mutations.

This means one cannot have a trait that is actually named *_items and have it synchronized:

class MyPreferencesPage(PreferencesPage):
    name_items = Str()

If this cannot be supported, we should document it and perhaps emit a warning when the trait is skipped.