#2956 .. is silently accepted in widget paths

[Jeff Rogers]

A child widget of the main application window can be created with a leading ".." in the pathname; this is silently interpreted as "." and the widget creation command returns the fixed path.

While I think this behavior is sensible - it simplifies a generic create a child window operation to "set widget_cmd [label $parent.name]" - the behavior is apparently undocumented. The behavior should either be documented/supported so that it can be relied on, or eliminated so that it does not become entrenched when people accidentally discover it.