Enable directory exclusion by database.ignore files

[Exeleration-G]

Request:

Current behaviour: set music directory in Noise --> Noise imports all subdirectories into its library
Desired behaviour: set music directory in Noise --> Noise imports all subdirectories into its library, without directories with an empty 'database.ignore' file in it.

The advantage of this would be that users can exclude podcasts directories from getting indexed in Noise. Or maybe there's music in their Music directory that they just don't want to have indexed.

Of course, users can selectively pick the music directories instead, but the auto-indexing is a nice feature in Noise.

I don't really mind whether the file is called database.ignore or something else, it's about the principle.