We need a way to be able to override variables from xwikivars.vm / layoutvars.vm without overriding these templates

It is a major pain to have to go through a custom skin to override variables from xwikivars.vm or layoutvars.vm, plus overriding those two templates means a merge need every time the wiki is upgraded.

A first improvement could be to add a #template("extravars.vm") at the bottom of xwikivars or layoutvars, and to keep this template empty on purpose in the standard distribution. This would fix the upgrade issue.

Next step could interface extensions; (or something like scripting extensions here, since we don't alter the rendered DOM, but just want to execute some velocity code at a certain point in the flow)