fix #1313 template error in _onTplLoad

If the disposal of a template is called before its loading, the `_onTplLoad` method could still access some not-disposed objects.
For this reason, the protected `_deleteTplcfg` method of `aria.widgets.Template` has been shared with and used by `aria.html.Template` through the class `aria.templates.TemplateTrait`

close #