Add support for speech recognition

[mrosso10]

Hi, I'm currently using https://github.com/openai/whisper to convert audio to text, and it would be nice to have it integrated to nchat.
I'm currently experimenting with the nchat code and I think I could start a PR with a "minimum viable feature", that would consist of:

• A shortcut that (having an audio message selected) spawns an external (configurable) process
• That process (whisper) would transcribe the audio and save the output to the same folder the audio is in (~/.config/nchat/profiles/...), with .txt extension
• A change to uihistoryview.cpp for rendering the audio transcription near the attachment file name.

Other improvements that would be nice:

• An sign near the attachment filename that indicates if the file is being processed
• Saving the transcription in the chat export

[d99kris]

Hi @mrosso10 - it's a cool idea, but at the same time I'm quite hesitent to adding corner-case features that increase maintainence work (for the core functionality). I suppose we can see how well encapsulated the functionality becomes and then might can consider merging such a PR, but I cannot promise it.