Give warning about RAP verification

[msg7086]

@Asd-g @realfinder
Bringing this up because earlier today I put lots of effort into debugging the performance issue until I see the bug report 2 weeks ago about the change. I think this is a pretty big change that is worth mentioning as the performance penalty is quite significant on certain type of files.

On my NAS server, a random m2ts file takes 11 seconds to index with verification off, but takes 160 seconds with verification on. I can imagine that the numbers would go further up when working on 4k or 8k videos.

Shall we at least add a warning message before showing the index progress, to warn user, in both on and off case? If it's on, warn user that it will be very slow, and if it's off, warn user that it may be inaccurate.