add mk_dragable

lib/slippery_slidy.mli

@@ -5,9 +5,13 @@ type datalist =
   ; datalist_el : El.t
   }
 
+(** - [id] is the id of the actual_slider_el element
+    - [el] is the slider container element
+    - [actual_slider_el] is the input slider *)
 type slider =
-  { slider_id : string
-  ; slider_el : El.t
+  { id : string
+  ; el : El.t
+  ; actual_slider_el : El.t
   }
 
 (** Any is for continuous slider *)
@@ -34,3 +38,6 @@ val mk_slider :
   -> slider
 
 val add_slider_input_listener : slider -> (float -> unit) -> unit
+
+(** setup listener to drag and move slider with mouse *)
+val mk_dragable : slider -> unit