Update name of a tscn when save as… is pressed.

[shahriarlabib000]

If you have a saved scene but you want to save it as a different name godot will currently show the previous name of the root node in the file dialogue popup. I think it should automatically suggest the new name in the file dialogue.
If we implement this…
Do you want the path to be same as the previously saved .tscn?
Or should it be suggested in the root folder?
Suppose i have a tscn named main .
Its saved at res://a/b/main.tscn I have renamed the root node to notMain
When using save as… do you want the suggested path to be

1. res://a/b/not_main.tscn
2. res://not_main.tscn
3. res://a/b/main.tscn
   Currently in the master branch it should suggest 3. res://a/b/main.tscn

[shahriarlabib000]

Checkout WIP godotengine/godot#100430