Init progress bar on Library panel (close #49)

Introduce two new callbacks for this: one when initializing the loading
of albums and another one on handling the loading of each album. Use the
first one to initialize and the second one to pulse the progress bar on
the Library panel.

Additionally use the text of the progress as status label instead of a
separate label widget.

mcg/client.py

@@ -110,6 +110,10 @@ class Client(Base):
     SIGNAL_STATUS = 'status'
     # Signal: stats
     SIGNAL_STATS = 'stats'
+    # Signal: init loading of albums
+    SIGNAL_INIT_ALBUMS = 'init-albums'
+    # Signal: pulse loading of albums
+    SIGNAL_PULSE_ALBUMS = 'pulse-albums'
     # Signal: load albums
     SIGNAL_LOAD_ALBUMS = 'load-albums'
     # Signal: load playlist
@@ -498,9 +502,12 @@ class Client(Base):
 
     def _load_albums(self):
         """Action: Perform the real update."""
+        self._callback(Client.SIGNAL_INIT_ALBUMS)
         self._albums = {}
         # Albums
         for album in self._parse_list(self._call('list album'), ['album']):
+            self._callback(Client.SIGNAL_PULSE_ALBUMS)
+
             # Album
             album = self._extract_album(album)
             self._logger.debug("album: %r", album)