Allow to adjust the `$in_admin` property of `WP_Screen`

[flixos90]

The $in_admin property of the WP_Screen class is impossible to be changed by a developer at the moment, which makes sense in how WordPress uses it.

However, I'm currently building a plugin that creates an additional administration panel (like network and user admin) and I would need to adjust the $in_admin property to reflect when this panel is active. Otherwise a function like is_blog_admin() returns true although we actually aren't in the regular admin panel (since the value of $in_admin defaults to 'site').

So I'm proposing to either make the property public so that a developer can override it (for example on the current_screen action) or to add a new filter for the property specifically in WP_Screen::get(). I would prefer the second approach since having the property being protected makes sense in general. The use-case I have is a very rare one, so I think a very specific filter for it should do.

[flixos90]

37445.diff​ introduces such a filter. I'm not sure about whether the name of that filter is the best possible choice though, so feel free to come with something more accurate.

[flixos90]

For reference: Implementing the proposal from #37526 would make this ticket invalid/unnecessary.