Added dictionary mapping names to children, and _name attribute.

	Added dictionary mapping names to children, and _name attribute.
	Renamed tkerror to _tkerror, default_root to _default_root.
	(_substitute, _subst_prefix): move back into class Misc; added
	widget Event attribute.
	Added functions that return widgets or widget lists:
	winfo_children; focus_get; grab_current.
	(Pack.slaves, Place.slaves): return Widget objects instead of
	pathnames.
	Renamed __del__() to destroy() (because of circular refs, __del__
	won't ever be called).
	Added some names with explicit _set: focus_set, focus_default_set,
	grab_set.
	Added focus_default_none.
	(Misc._getints): use tk.splitlist() instead of tk.split(), return
	None if string empty.