Update database after tags are edited

For some reason nobody updated the database when tags where edited.
Problems:
- When editing many songs the Tagedit dialog stays open longer (which I think is ok).
- If there are calls to TagReaderClient::SaveFile (non blocking) which expect to return really fast we might not return as fast. There are some places where this is called and I could only test Inline Editing which is fine with this change.
- I didn't test CDRip.


This might fix: #4419, #221 but I only looked at the description if this might be what the reports mean.
P.S.: Initially I tried to use NewClosure to call the new function but I couldn't make it work. If somebody wants to take a stab at it, be my guest.
You shouldn't call this until you're sure "Save file" worked.
But I believe this is what you tried to do with NewClosure right? This is probably the best way to go indeed: use closure to connect the reply->finished signal to SongSaveComplete (which should be a slot) and check their that reply has succeed (otherwise we shouldn't update the DB).
Having a look at the other places in the code where we use NewClosure didn't help you to figure out what was wrong in your case?