Support captions for the gallery macro images

See XWIKI-6762 .
The support for alt was added, but the gallery still does not support captions.

"""
Any chance of extending the Gallery Macro so that you could show a caption for each image? Perhaps use the alt text as the caption. My thinking is using the gallery as part of a step-by-step tutorial where you flip between the images and the captions describes what you should be doing. This would make the images more meaningful rather than a beauty pass gallery (which looks great by the way).

I guess something like:

{{gallery}}
[[image:iphone.safari.settings.png||alt="Open the settings page for Safari"]]

[[image:iphone.safari.setting.cookies.png||alt="Ensure that Javascript is turned on and Cookies is set to From Visited"]]
{{/gallery}}

I am looking at using XWiki for the client-side Documentation of our webapp and am enjoying it so far.
"""