Media handling improvements (potential followups to #1223/#1058/...)

[neiljp]

Areas which we could explore: (post #1223)

• ensure it works reliably on all the platforms it was designed for
• better encapsulate platform-specific code (at least one location uses the PLATFORM variable outside of platform_code.py)
• is there a redundant adjustment of the path for WSL?
• support option to 'copy URL' from message information popup (don't download file)
• support option to 'copy path' after download (eg. for handling by another preferred tool)
• support custom open locations?
• use more widely supported download (or cache?) location?