feat: Add option to get the current slide.

[immadisairaj]

Problem

We know that the PresentationWidget handles the movement of slides. But, when want to make a small customisations to the background widget (like simple background animations) or anything, there can be a need for getting what slide the presentation is currently in.

Proposal

An option to attach a listener to the PresentationWidget and listen to slide changes that gives us what slide we are currently in.

Additional Details

The background of PresentationWidget is being appended always as the whole slides are in stack. So, there might not be a possibility to have background animations or even if they are, it will cause performance issues. So, we need to think of something like having Opacity to slides so to have only one background of PresentationWidget instead of multiple copies of it for each slide.

[immadisairaj]

This feature can also give possibility to animate a nth slide (or content inside) when moving to n+1th slide. This can be done with both this and #7.