Fix autoplay and automatic start of animations

[MX-Futhark]

A certain number of bugs was included in the animator plugin, among which:

• consecutive slides (more than one) with an empty animator prevented autoplay from continuing;
• animations that should start playing automatically when their slide is displayed had to wait for an input from the user;
• the "deck.animation.sequence.stop" event was triggered too many times in specific circumstances, leading to errors in the folowwing animations;
• the autoplay function was not always disabled when it was supposed to.