The Godot editor is actually a Godot project, but you won't be able to use the handles from the editor directly. You'll need to make your own handles.
One way to do this is to make a panel, set it to the size of the texture, and then give a StyleBoxFlat with the center not shown. Anchor a bunch of TextureButtons to the sides and corners of the panel. Detect clicks on the buttons and change the size of the panel to be where the mouse has moved. Also change the size of the texture as well.