Center aligned image and audio

[rfohlin]

trafficstars

[otacke]

@rfohlin I am not a member of the H5P core team, but I'd like to point out one issue that might need your attention: Using your changes, the visual design of all existing content would change. I don't think you can assume that everyone wants images to be centered, so this might require a switch in the editor at least. However, I think implementing https://h5ptechnology.atlassian.net/browse/HFP-1926 (in H5P.Column) would be the proper way to tackle this.