5.8.0: improved virtual tour & elements markers

[mistic100]

In this version the virtual tour plugin gets a big overhaul with a new rendering of navigation arrows. Also the elementLayer marker allows to integrate complex elements like an iframe (eg: Youtube embedded video) in your viewer.

Check the new virtual tour arrows here https://photo-sphere-viewer.js.org/plugins/virtual-tour.html#example

Check the new element marker here https://photo-sphere-viewer.js.org/demos/markers/youtube-element.html