Allow cancellation when saving as externally modified file

[Vectornaut]

Right now, if I try to save an editor tab into a file that's been modified externally, I'm presented with two options:

• Reload: Overwrite the tab with the contents of the file.
• Save: Overwrite the file with the contents of the tab.

If I don't want to overwrite anything, I have to choose a secret third option: pressing [escape] to dismiss the dialog. I want to present this option explicitly as a Cancel button in the dialog. I also want to remove the Reload button, because I never ever want to do this. If there are no objections, I'll implement this and make a pull request.