Previous and Next allow do not work properly if there are multiple timelines on the same page

[WilsonWu2100]

The current code only allows one timeline to be working on every page, the previous and next allow do not work properly if there are multiple timeline on a single page.

This fix will allow multiple timelines to be working properly on the same page.

[stefanomarra]

@WilsonWu2100 looking good to me, but can you update the code in the source folder? Once the change has been moved there you can run gulp to build the plugin in the dist folder

[WilsonWu2100]

@stefanomarra Changes added to source code.