The way we did this was to do the following:

In App_Plugins create a new folder containing a custom controller.js, custom.html file and a package.manifest

Then within the Umbraco source file locate the umbraco.controllers.js file and copy the function for the content picker, place this in your custom.controller.js file. Then, go back to the umbraco source folder and locate property editors folder, within here locate the content picker folder and html file. Copy this into your custom.html file.

Now within your package manifest put in something like so:

        {
            propertyEditors: [
                {
                alias: "My.CustomPropertyEditor",
                name: "MyCustomPropertyEditor",
                isParameterEditor: true,
                editor: {
                view: "~/App_Plugins/CharLimitEditor/charlimiteditor.html"
                },
    prevalues: {
                    fields: [
                        {
                            label:"Start node",
                            description:"Choose the default folder",
                            key:"startNodeId",
                            view:"mediapicker"
                        }
        ]
            }
        }
    ],

    javascript: [
        "/App_Plugins/List-Items/list-items-custom.controller.js"
    ]
}

Add in the prevalues and that's about it :-) Adding isParameterEditor: true, in the package manifest makes the option appear in the type dropdown field as you highlighted above.