Warn user on first metadata edit

[rexendevar]

I already mentioned how I used Deadbeef to mess up my entire 6000 song library's metadata all at once. The reason I did that was, of course, I didn't realize doing so was even a possibility. I had NO IDEA that Deadbeef would have to rewrite the full metadata stack for every individual file rather than simply adding one extra field. I don't think this should happen again!!!! I think Deadbeef should warn you, the first time you edit any metadata, that it will probably make a LOT more changes than you expect and that you should check the metadata settings before you start. Just one message box could have prevented my disaster from happening.