Fix wrong undo-redo action when dropping files containing circular dependencies

[luevano]

Found in #89106 (comment) originally.

When trying to drop files containing a scene that contains cyclic dependencies (such as dropping the scene into itself), a new undo-redo action "Create Node" will be created and a generic error will appear. This happens in both 2D and 3D as far as I'm concerned.

Behavior BEFORE change (tested @ 1b2e0b3):

godot.windows.editor.x86_64_2pBYJv8ecv.mp4

Fixed behavior in this PR:

godot.windows.editor.x86_64_AjeYE6YGVt.mp4

I have my doubts on how to communicate the "cyclical dependency found" (as well as the message itself), currently I'm using the same kind of messaging/notification as that of "Instantiating : ..." in the bottom left of the screen for both 2D and 3D. I experimented with showing a popup (like the one with the bug) with a custom message to let the user about the cyclical dep but requires a lot more changes and some hacks.

Also, not sure if the way I handle checking for cyclical dependencies are optimal, specially for the 3D version, basically all files should be checked until at least one cyclical dep is found, this causes more iterations in general.

Finally I decided to use _remove_preview/_node to remove the "cyclical dependency found" message once the dragging stops as it is being called in the notification, and decided to match the behavior of setting the message to "" in 2D always as how its done in 3D.

[luevano]

CC @KoBeWi; as you mentioned the possibility to modify _perform_drop_data instead (#89126 (comment)) but I found it making more sense (to me) to check in can_drop_data itself.

It feels more "natural" to do so in can_drop_data. Also when can_drop_data returns false, the cursor changes to a "forbidden cursor" thus giving more feedback to the user.

[luevano]

Rebased against 9b94c80 and applied the suggestions. @AThousandShips let me know if the way I handled showing the error file is fine #89204 (comment).

[luevano]

Rebased against aef11a1.

Applied #89204 (comment), #89204 (comment) and #89204 (comment)

[akien-mga]

Thanks!