Export to PDF should confirm that user wants to overwrite existing file

[mikkorantalainen]

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Is your feature request related to a problem? Please describe.
When I'm happy with the drawing and I choose File - Export as - PDF... and export the PDF file, the Save dialog doesn't complain if I choose name that collides with existing file but silently overwrites any existing file with the same name.

Describe the solution you'd like
The save workflow should show dialog "Warning: File foobar.pdf already exists." with dialog buttons "Overwrite" and "Cancel" where the Cancel action should return to the file selector instead of cancelling the whole export process. The cancel button in file selector should still cancel the whole export.

Describe alternatives you've considered
The export filename could be automatically silently modified (e.g. to add (2) before the extension) to avoid overwriting the target file but I think this would be even worse than the current situation because user might not notice this and wonder why the output file doesn't match the content the user was trying to export.

[alderg]

What operating system and version?