Fix bug 1789838 - Ask for confirmation before overwriting template
Ask for confirmation to user before overwriting an existing template.
How to check:
- Create a document with some drawing and save it as a template.
- Modify document and try to create a template with the same name.
- A dialog asking for confirmation must be displayed.
- Cancel overwriting and save the template with a new name.
- Modify the document and overwrite one of the created templates.
- Check the two templates contents with "New from Template...".