InstancePlaceholder improvements.

[CarpenterBlue]

So I feel there are quite few issues with InstancePlaceholder for being such a great tool.

1. upon setting load as placeholder, node loses it's script which necessitates adding another node with a script...
2. InstancePlaceholder also steals Metadata from the node it's replacing
3. It is not possible to discern that node is loaded as a placeholder from the scene tree
4. If used with exported node, it also take over the path of the OG node.

How do you feel about these limitations?
I am somewhat OK with last two of them except the 1st and 2nd one but I do not have good solution for it.
maybe the node could have separate placeholder script and metadata?