rewrite populate.get_image() to extract a

track thumbnail from the first track in an album that has one.

+ rewrite Populate.remove_modified with sets
+ clean the SqliteManager utility class
+ Rewrite ProcessTrackThumbnails to use a process pool instead of a thread pool
+ rewrite track store's  remove_tracks_by_filepaths to utilize sets

app/lib/watchdogg.py

@@ -92,7 +92,7 @@ class Watcher:
             )
             return
         except OSError as e:
-            log.error('Failed to start watchdog. %s', e)
+            log.error("Failed to start watchdog. %s", e)
             return
 
         try: