[cmake-developers] DragNDrop: Easier custom dmg layouts

[Robert Maynard]

I have pushed to next a branch called better_custom_dmgs that improves
and helps automate the process of creating custom dmg's.

Previously the DragNDrop generator allowed the user to specify a
custom background image file and a custom .DS_Store. This approach had
some serious limitations the primary being that a new .DS_Store would
have to be externally created each time the package name was changed.

The new version now allows the user to specify a custom apple script
to run during the package step that is used to generate the .DS_Store.
This allows projects to now have a nice looking package without having
to externally generate a new .DS_Store.

Additionally this branch has changed the behavior of
CPACK_DMG_BACKGROUND_IMAGE. Now the background image is stored in a
hidden folder, and the extension of the source image is retained.
These changes are needed to allow the custom apple script to easily
set the background image.

If anyone has any questions or issues with this branch please tell me.