adds listing_images meta box to properties admin screen

[charlesberg]

listing_images was not previously added to the meta_boxes array, however all other necessary methods for displaying the meta box and saving data were implemented.

[JoeHana]

Hey :)

This gets actually added from theme since we've considered this to be theme-specific. That being said, this modification will enable the metabox in the admin ui but would still lack of a proper output.

But thanks a lot for your efforts so far

[charlesberg]

Ok understood. At least it seems to output the correct post meta (_gallery) to the db... I will extend from a child theme instead.

Cheers!