Make gallery properties publically accessible.

They were public in the previous release of MediaWiki (1.22), although
the visibility was not set explicitly.

See I779d5a683982dc25bc7cdd30addd5870f2a7efd0 and
https://github.com/SemanticMediaWiki/SemanticResultFormats/issues/12

Change-Id: I309cf46891223c5f1520e97b04d4ed315a300e7a
(cherry picked from commit d7f494c7be0718fef0f763e7a4286fcaf6a476c2)