Fix closing Theme Editor not actually closing it

[KoBeWi]

Fixes #97278

The issue has uncovered a bug in editor closing logic. Folding a resource will call the editor to stop editing it, but the actual unediting code assumed that the EditorPropertyResource no longer exists (which can happen when inspector updates, e.g. when editing another object). Which means the resource wasn't actually unedited.

I wasn't sure how to solve it properly, so I just added _should_stop_editing internal method. It's meant for editor contexts to inform whether they are still editing the object or not. EditorPropertyResource will return true when it's folded, so folding a property now properly stops editing it.

Then there is ThemeEditorPlugin, which used the self-owning mechanism added in #81523, but the method for determining whether the editor should be hidden was overly complex and assumed that the Theme is opened in the inspector. I changed it to simply stay open while it edits a valid theme. Which means that Theme Editor will keep Theme reference until you explicitly press Close button; it no longer automatically disappears. Not sure if that's expected behavior, but it's at least convenient 🤔

The remaining problem is that when you close the scene, a built-in Theme can still remain referenced, resulting in potential data loss. Not sure about a solution for this one (maybe some WeakRef?), but it's probably fine for now. Solved it with theme_closed signal, which will automatically close editor if it edits built-in Theme.

[akien-mga]

Thanks!