a lot of changes to make videos a bit more robust

I still have a problem in lagging while moving the mouse though.

src/core/kinds.rs

@@ -5,9 +5,7 @@ use serde::{Deserialize, Serialize};
 use crate::Slide;
 
 use super::{
-    images::Image,
-    presentations::Presentation,
-    songs::Song,
+    images::Image, presentations::Presentation, songs::Song,
     videos::Video,
 };