Active slide event

[v-matheusb]

Added the glider-slide-active event to solve an issue where I discovered when making this type of slider:

It's composed of 2 sliders and I sync them by updating the current slider index during the glider-slide-visible event. I needed the centered slide to be active and adjacent ones to show partially, but I found that in some screen sizes, the event was triggering for one of the adjacent slides, which made it skip one slide in some cases, as well as sometimes causing glider-slide-visible to trigger multiple times while scrolling to a slide.

My solution was to create the glider-slide-active and made sure that it would only trigger once per slide scroll.