Expose dialog parent-and-popup logic to the API

[YuriSizov]

This was introduced in a hacky way before to fix immediate issues with some of the editor windows (see #73365). But I think we should have this feature generally accessible, so we can address similar issues in other editor windows, and so plugin developers could use it too. In fact, this is probably useful to user projects as well, if they support multiple windows.

So this removes the old hacky class and moves the necessary functionality to other places.

• EditorInterface receives new popup_dialog_* methods to automatically reparent a window to the editor UI before popping it up.
• Window receives set_unparent_when_invisible, a new scripting-only method to enable the "unparent this window when visibility changes to not visible" logic. It's scripting-only because using it requires you to have a script, and enabling it by accident can lead to orphan nodes. Window also receives new popup_exclusive_* methods, which power EditorInterface methods secretly.
• Node receives get_last_exclusive_window, which tries to find the top-most exclusive child of the owner window, or returns the parent window (or nothing, if there is no window); this can be exposed as a part of Window as well, but being a part of Node makes it easier to use, as it can handle the owner window internally and return a clean result.

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Marking as a draft as this is based on the EditorInterface refactoring from #75694.

[YuriSizov]

Since relevant parts were extracted with #76176, which is merged, this is now rebased on top of master and ready for review.

[YuriSizov]

@bruvzg I added popup_exclusive_* methods to Window, but kept special EditorInterface methods as well because they require a reference to EditorNode, and it just makes using the feature within the editor and plugins easier.

[KoBeWi]

I wonder if popup_exclusive() is accurate name, because dialogs are exclusive by default (otherwise we wouldn't be getting exclusive errors).

Other than that looks fine.

[YuriSizov]

I wonder if popup_exclusive() is accurate name, because dialogs are exclusive by default (otherwise we wouldn't be getting exclusive errors).

That's a good point. The idea behind the name is that it ensures that exclusivity is respected, but perhaps it's not clear. Don't have a better suggestion myself at the moment. I don't think it's a misnomer though.

[akien-mga]

Thanks!