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:

• Concat/merge files
• Fix invalid duration
• Capture video frame as image
• Capture video frames as images
• Extract track as file Prompt file overwrite when extracting audio track #2053