core: Add last_modified to track model
local: start reading mtime from tag caches
scanner: Add last_modified to scanner data
scanner: Pass in files to scan
scanner: Load existing tracks from local library.
scanner: Respect mtime when picking files to scan.

Speedup scanning by checking what files exist, and which have been updated.
scanner: Update based on review comments.
Merge pull request #452 from adamcik/feature/mtime

Add last modified time to tracks and make scanner mtime aware.