Optimize indexing of files

[berarma]

I was sitting on this waiting for some word on #954. Since there's already some other work going on for the same goal, I'm posting it in case you want to use it before it gets lost.

It's based on a commit prior to the PR that caused the graphical issues because I wanted a fully working version. The current releases don't work for me. It should be easy to apply to master in its current state.

In my computer there's a close to 3x improvement in the creation of the cache and even more when updating it. A lot of time is still being used to look up the songs in the database. Merging it with the cache would yield another good improvement.