Fix modal dialog callback being called multiple times

When the modal closes, we need to remove the DOM element. 
If not, we create a new EventListener associated with the modal each time the modal shows up.
And basically when the user `Cancel` the modal, all the previous callback where the modal was used get called.
That's pretty critical

src/app/ui/modaldialog.js

@@ -82,6 +82,7 @@ module.exports = (title, content, ok, cancel, focusSelector, opts) => {
 
   function hide () {
     if (container) container.style.display = 'none'
+    if (container.parentElement) container.parentElement.removeChild(container)
   }
 
   function show () {