This fixes the plugin tpl fallback for all plugins, already using the parseTemplate() method. All others, which may still follow the themes fallback (like contactform etc), would need to always be part of the user template $serendipity['template'], or be fixed later on.
This also fixes the backend chaining, which now simply follows the force with a possible engine and then uses $serendipity['template_backend'] (2k11), $serendipity['defaultTemplate'] (2k11), 'default'.
As a third, this now uses the correct preview_iframe.tpl file on save and checks for a correct set jquery_backend.js in the user theme $serendipity['template'].
Please double check this approach for cases I did not find yet. Thanks! :)
References #343
d5eae21a9c