Exported Script-type resources will now be treated properly as Scripts in EditorPropertyResource

This makes the EditorPropertyResource work properly with exported Script-type resources (that is, properties marked with export(Script)).

By default, the ScriptCreateDialog will default "Inherits" to Resource. Except, of course, when the property already has a script, where it will default "Inherits" to that script.

By default, the ScriptCreateDialog will default "Path" to <node_or_scene_name>.<property_name>. (with a trailing period to prevent the property name from being seen as an extension), with the proper extension being added onto the end by the ScriptCreateDialog (as it already does).

This is a draft because I'd like to make sure everything is in order before it's merged. Please read the review comments below and give me any responses to them that you might have! When the PR is ready for review, I'll merge all the commits and remove the comments I've left behind.