Timeline Group Visibility

Resubmit of #2303. I chose not to completely remove modifications to the library but instead incorporate the method @yotamberk described in #2303 into Timeline.setGroups. This PR will automatically filter groups where group.visible == false. This is more efficient than what I had proposed in #2303 in that the hidden groups do not get rendered, however it still provides a simple imperative means for the library user to hide groups.