Expose internal editor controls used in the Inspector dock

[samdze]

Describe the project you are working on:
Godot editor plugins.

Describe the problem or limitation you are having in your project:
When I want to design custom editors for the inspector or for custom docks it is very annoying and time consuming to reimplement the functionality that Godot already provides in the inspector or in other parts of the editor.
This is one of the points I listed here: #300

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
Expose the editor nodes and types, like the subclasses of EditorProperty, various dialogs/containers (to select a property from a node or resource, to select a method, to create a Node/Resource, etc) in the Create Dialog window when needed.
There can be a switch to enable editor nodes or something like that.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
When the editor nodes are active in the Create Dialog, they could easily be created inside a scene to quickly design custom editors and UIs that behave the way the default Godot ones do.

If this enhancement will not be used often, can it be worked around with a few lines of script?:
Editors can be created in scripts, but it is a very long and unintuitive process.
Nodes are better organized and configured inside the scene editor.

Is there a reason why this should be core and not an add-on in the asset library?:
It enables more tools to better design future add-ons.

[Arrow-x]

https://github.com/Shfty/inspector-gadget might help a bit, it is more Inspector stuff, but a good start.
but I agree exposing more of the work already done sounds like a good Idea