Show album length on Playlist and Library panel (close #57)

Show the length of an album in the tooltip on the Playlist and the
Library panel.

mcg/utils.py

@@ -66,6 +66,13 @@ class Utils:
         return label
 
 
+    def create_length_label(album):
+        minutes = album.get_length() // 60
+        seconds = album.get_length() - minutes * 60
+
+        return locale.gettext("{}:{} minutes").format(minutes, seconds)
+
+
     def create_track_title(track):
         title = track.get_title()
         if track.get_artists():