TwingTemplate::loadTemplate fails to recognize valid templates if they are not of class TwingTemplate

A template is not an instance of TwingTemplate. It is something that have the expected public shape - and mainly some render and display functions. TwingTemplate::loadTemplate needs to be fixed to take this into account. By enforcing that a template is of class TwingTemplate, it prevents using objects that were not instanciated by the TwingTemplate factory but are totally legit templates.

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