Add appearOnClick property to images/shapes/media

[MelleB]

Hi,

Thanks a lot for this useful library. I was missing an option to show objects on click, so I hacked this together to suit my needs. I am aware that this is technically an animation and not part of the scope of this package, but maybe this work benefits someone else and/or provides someone inspiration to add full-fledged animation capabilities to this library.

Again, thanks for this package.