Visually indicate changes of playlist and library (close #42)

Set the GTK “needs-attention” property of the header bar stack switcher
for the playlist and the library panel when their content changes.

mcg/client.py

@@ -115,6 +115,8 @@ class Client(Base):
     SIGNAL_LOAD_PLAYLIST = 'load-playlist'
     # Signal: load audio output devices
     SIGNAL_LOAD_OUTPUT_DEVICES = 'load-output-devices'
+    # Signal: custom (dummy) event to trigger callback
+    SIGNAL_CUSTOM = 'custom'
     # Signal: error
     SIGNAL_ERROR = 'error'
 
@@ -275,6 +277,11 @@ class Client(Base):
         self._add_action(self._set_volume, volume)
 
 
+    def get_custom(self, name):
+        self._logger.info("get custom \"%s\"", name)
+        self._add_action(self._get_custom, name)
+
+
     # Private methods
 
     def _connect(self, host, port, password):
@@ -604,6 +611,10 @@ class Client(Base):
         self._call('setvol', volume)
 
 
+    def _get_custom(self, name):
+        self._callback(Client.SIGNAL_CUSTOM, name)
+
+
     def _start_worker(self):
         """Start the worker thread which waits for action to be performed."""
         self._logger.debug("start worker")