Allow adding a custom class to the gallery wrapper

[davekiss]

https://wordpress.org/support/topic/need-custom-classes/

While it is nice to duplicate the theme settings,
what we need is ability to set a custom class.

I will have many many many galleries.
I customize the css for two different galleries
but some galleries have CSS style A and some B.

I need a way to differentiate the gallery themes and css
without having to hard code every gallery number

please

also, custom class should copy with other theme settings

[davekiss]

We may eventually build out a UI control for this in the admin, but the related commit can close this issue for now.