Allow additional contents for DMG

Summary

Create a macosx specific jpackage option to allow adding additional content to a dmg created by jpackage

Problem

Jpackage will create a DMG consisting only of the app-image.

Customers have requested ability to add additional content to the DMG

Solution

Add a jpackage option to allow adding arbitrary content (files and/or directory trees) to the dmg.

Specification

Add the new option:

--mac-dmg-content <path to additional content>[,<path to additional content>...]
Include all the referenced content in the dmg.
This option can be used multiple times.

Only valid for Mac dmg type bundles, Usage with other bundle types is an error.

The mac-dmg-content argument is a comma separated list of paths to any content (presumably a runtime image or pkg, but can be anything)

The Disk Image created by jpackage will include the application plus all the mac-dmg-content. It's dmg script will show in finder the main app image and prompt to move it to the install-dir (as now) but will also show all the co-bundled items.