when audio file referenced by CUE file doesn't exist, show a file dialogue

[llfw]

Is your feature request related to a problem? Please describe.

often, people release a CD rip as a CUE file and an image in a compressed format like FLAC, APE, etc., but they don't update the CUE file to reference the compressed file, so it references the ".wav" file instead. CUE file in such release cannot be opened by fre:ac.

Describe the solution you'd like

when a CUE file is valid, but the audio file is not found, then instead of reporting that the CUE file cannot be opened, instead offer an option to open a file selection dialogue to select the correct audio file.

Describe alternatives you've considered

an alternative would be to select files in other formats automatically, e.g., if the CUE file references 'CDImage.wav', but 'CDImage.flac' exists, automatically open CDImage.flac instead. however, i think this might be too error-prone.

Additional context

this issue is very common in pirate music releases, so it would save a lot of time if fre:ac could handle this situation better.