Filter/toggle to hide nodes in tree that have no variant in the current language

[Luuk1983]

Inspired by this bug: #19102, I really felt we need to have a toggle or filter or something in that direction that lets you hide the nodes in the tree that are not available in the current language.

This is just some mad paint skills for the idea, it's too ugly for production obviously ;)

But especially on large sites, the tree can get really clogged with nodes that do not have a variant in the current language. Yes, I know the idea is that every node should have a variant in the default language, but in reality that's just not always simply the case.