templatefilters: fix crash by string formatting of '{x|splitlines}' Before, it crashed because mapping['templ'] was missing. As it didn't support the legacy list template from the beginning, we can simply use hybridlist().

templatekw: factor out showdict() helper Make it less cryptic for common cases.

templatekw: have showlist() take mapping dict with no **kwargs expansion (API) See the previous commit for why. splitlines() does not pass a mapping dict, which would probably mean the legacy template didn't work from the beginning.