Config: Make CoreConfig a singleton object

This prevents the configuration from being read and modified in two different
places (`CoreConfig` objects are created in `datastore.py`, `treefactory.py`,
and `backends/__init__.py`).

* GTG/core/config.py (CoreConfig_): Rename from `CoreConfig`.
(CoreConfig_.INSTANCE): New field (use it to store the unique config instance).
(CoreConfig_.__init__): Make sure that we are only ever initialized once.
(CoreConfig_.get_instance): New method.
(CoreConfig): New function to initialize and return the singleton config object.

GTG/core/config.py

@@ -180,10 +180,14 @@ def save(self):
         self._save_function()
 
 
-class CoreConfig():
+class CoreConfig_():
     """ Class holding configuration to all systems and tasks """
 
+    _instance = None
+
     def __init__(self):
+        assert self._instance is None
+
         self._conf_path = os.path.join(CONFIG_DIR, 'gtg.conf')
         self._conf = open_config_file(self._conf_path)
 
@@ -193,6 +197,11 @@ def __init__(self):
         self._backends_conf_path = os.path.join(CONFIG_DIR, 'backends.conf')
         self._backends_conf = open_config_file(self._backends_conf_path)
 
+    @classmethod
+    def get_instance(cls):
+        cls._instance = cls._instance or cls()
+        return cls._instance
+
     def save_gtg_config(self):
         self._conf.write(open(self._conf_path, 'w'))
 
@@ -231,3 +240,6 @@ def get_backend_config(self, backend):
             self._backends_conf[backend],
             DEFAULTS['backend'],
             self.save_backends_config)
+
+def CoreConfig():
+    return CoreConfig_.get_instance()