Add send_art event for embedart and thumbnails

Album.set_art() sends a 'art_sent' event, with the album as a
parameter. embedart and thumbnails listen to that event, instead of
listening to 'album imported'. Consequences:
- 'embedart' and 'thumbnails' don't have to be after 'fetchart' on the
  plugins config line.
- embedart and thumbnails work event when a "beets fetchart" command is
  issued.
- if another plugin ever set art then embedart and thumbnails will "just
  work" with it.

beets/library.py

@@ -960,6 +960,8 @@ class Album(LibModel):
         """Sets the album's cover art to the image at the given path.
         The image is copied (or moved) into place, replacing any
         existing art.
+
+        Sends an 'art_set' event with `self` as the sole argument.
         """
         path = bytestring_path(path)
         oldart = self.artpath
@@ -983,6 +985,8 @@ class Album(LibModel):
             util.move(path, artdest)
         self.artpath = artdest
 
+        plugins.send('art_set', album=self)
+
     def store(self):
         """Update the database with the album information. The album's
         tracks are also updated.

beetsplug/embedart.py

@@ -53,7 +53,7 @@ class EmbedCoverArtPlugin(BeetsPlugin):
             self._log.warning(u"ImageMagick 6.8.7 or higher not installed; "
                               u"'compare_threshold' option ignored")
 
-        self.register_listener('album_imported', self.album_imported)
+        self.register_listener('art_set', self.process_album)
 
     def commands(self):
         # Embed command.
@@ -106,10 +106,10 @@ class EmbedCoverArtPlugin(BeetsPlugin):
 
         return [embed_cmd, extract_cmd, clear_cmd]
 
-    def album_imported(self, lib, album):
-        """Automatically embed art into imported albums.
+    def process_album(self, album):
+        """Automatically embed art after art has been set
         """
-        if album.artpath and self.config['auto']:
+        if self.config['auto']:
             max_width = self.config['maxwidth'].get(int)
             self.embed_album(album, max_width, True)
 

beetsplug/thumbnails.py

@@ -52,7 +52,7 @@ class ThumbnailsPlugin(BeetsPlugin):
 
         self.write_metadata = None
         if self.config['auto'] and self._check_local_ok():
-            self.register_listener('album_imported', self.imported)
+            self.register_listener('art_set', self.process_album)
 
     def commands(self):
         thumbnails_command = Subcommand("thumbnails",
@@ -68,9 +68,6 @@ class ThumbnailsPlugin(BeetsPlugin):
 
         return [thumbnails_command]
 
-    def imported(self, lib, album):
-        self.process_album(album)
-
     def process_query(self, lib, opts, args):
         self.config.set_args(opts)
         if self._check_local_ok():