Slider: set timeoutID to `undefined` on remove

... fixing interference with other timeouts where the same id was assigned to
another timeout that would be deleted erroneously. This was tracked down
because a timeout in the `spotrem` version being developed alongside
this slider module would stop executing code.

modules/Slider.js

@@ -131,6 +131,7 @@ try { // for making it possiblie to run the test app in the following catch stat
       o.f.remove = ()=> {
         Bangle.removeListener('drag', o.f.dragSlider);
         o.v.dragActive = false;
+        o.v.timeoutID = undefined;
         cb("remove", o.v.prevLevel);
       };
     }