Change preview methods to take Callable

[KoBeWi]

In Godot 4 most methods moved from "object, method, userdata" combo to just use a single Callable. The only remaining one (I think) are preview methods: queue_resource_preview and queue_edited_resource_preview.

For now the new methods are unexposed, and only used internally.

[Mickeon]

Could we change the implementation internally but still keep the original method exposed as is?
The current state of things is obviously an oversight, but the current name feels so right that any newly exposed method would be unusable just by its confusable nature. When it's gonna be time to clean the slate and remove all deprecated methods, the newer, yet "badly" named methods may slip through the cracks.

At the end of the day, fortunately, it's pretty easy to mitigate this issue by fetching a Callable's object and method name individually.

[KoBeWi]

You mean like, not exposing the new methods? Yeah, they don't need to be exposed. I mostly noticed it when doing #108147

[Mickeon]

Indeed, maybe slapping a huge "TODO" to have those methods use Callable as soon as it's reasonable to do so.

[Mickeon]

Now that the change is internal this is more reasonable.

That aside, the PR is a pretty sweet clean-up of a very shoddy, likely legacy, part of the editor.
It may speed up things a little too, but that was not the goal. Seeing Variant being passed around for no reason was already painful enough.