Fix:GcodeEditor close issues

Summary:
Before when you closed files sometimes the wrong one would be closed.
This fixes that by tracking the document itself instead of its index in the list

Reviewers: laysrodrigues, tcanabrava, patrickelectric

Reviewed By: laysrodrigues

Subscribers: Atelier: General Development

Tags: Atelier: General Development

Differential Revision: https://phabricator.kde.org/D13126