Fix not being able to reopen signals panel immediately after connecting a signal

[Hilderin]

Fixes #92996

The issue was that last_valid_node was being set to null when p_node was null, causing the loss of the last valid node. After a signal connection, restore_last_valid_node is called, but last_valid_node no longer contained the node being edited.

[KoBeWi]

Something is wrong. The Node dock shows the connecting node, but the inspector does not.

iiyWwNaQ5q.mp4

[Hilderin]

I changed a couple of things because it's was weird that the NodeDock and Inspector were not working the same way.

I changed the fix that was made from #84530 simply by add the line NodeDock::get_singleton()->set_node(nullptr); in scene_tree_dock.cpp when the node is deleted.
Also, I added a previous_item method in EditorNode so the _add_callback could select back the last node.
That allowed me to remove everything managing the last valid node in NodeDock.

[KoBeWi]

Note that restore_last_valid_node() was added in #54071 to fix the same issue, but seems like it no longer works correctly for some reason.

[akien-mga]

Thanks!