Dropping field outside of the form deactivates hover state.

When you drop a field on anything except a droppable the hover states (displaying the pen and bin) stop working.

Steps to reproduce

1. Create a webform.
2. Drop two textfields into the form.
3. Drop one of the textfields onto the other.
4. Hover the mouse above one of the fields.

Expected result

The hover state is triggered as normal, displaying the pen and bin symbols.

Actual result

No hover state is triggered.

Cause

The only place where Drupal.form_builder.activeDragUi is unset is Drupal.form_builder.dropElement. This function is only called when the drop is accepted by the droppable.

Proposed solution

Unset Drupal.form_builder.activeDragUi in Drupal.form_builder.stopDrag.