Prevent overwriting files

[mifi]

With the main export function, files will only be overwritten (with a warning) if the user has selected the overwrite option (default enabled). However there are other cases where a file currently gets overwritten if it exists, and we should probably give warning or refuse to output if that's the case:

• [x] Concat/merge files
• [ ] Fix invalid duration
• [ ] Capture video frame as image
• [ ] Capture video frames as images
• [ ] Extract track as file #2053