Index backends by id, not by name

This allows GTG to have multiple backends of one kind at the same time.

* GTG/backends/__init__.py (BackendFactory.get_saved_backends_list): Rename
`backend` to `backend_id` for clarity.  Call `config.rename_backend_section` on
each backend for compatibility with previous versions of GTG.
* GTG/core/config.py (CoreConfig_.rename_backend_section): New function to
rename a config section (from backend name to backend ID).
(CoreConfig_.get_all_backends): Rename `backend` to `backend_id` for clarity.

Fixes getting-things-gnome#930.

GTG/backends/__init__.py

@@ -138,8 +138,8 @@ def get_saved_backends_list(self):
         config = CoreConfig()
         backends = []
 
-        for backend in config.get_all_backends():
-            settings = config.get_backend_config(backend)
+        for backend_id in config.get_all_backends():
+            settings = config.get_backend_config(backend_id)
             module = self.get_backend(settings.get('module'))
 
             # Skip this backend if it doesn't have a module
@@ -167,7 +167,11 @@ def get_saved_backends_list(self):
                     # Parameter not found in config
                     pass
 
-            backend_data['backend'] = module.Backend(backend_data)
+            backend = backend_data['backend'] = module.Backend(backend_data)
+
+            # Rename configuration sections created by older versions of GTG
+            config.rename_backend_section(backend.get_name(), backend.get_id())
+
             backends.append(backend_data)
 
         return backends

GTG/core/config.py

@@ -229,16 +229,25 @@ def get_task_config(self, task_id):
             DEFAULTS['task'],
             self.save_task_config)
 
+    def rename_backend_section(self, backend_name, backend_id):
+        """Rename section `backend_name` to `backend_id` if it exists."""
+        if backend_name in self._backends_conf:
+            assert backend_id not in self._backends_conf
+            self._backends_conf.add_section(backend_id)
+            for (k, v) in self._backends_conf[backend_name].items():
+                self._backends_conf.set(backend_id, k, v)
+            self._backends_conf.remove_section(backend_name)
+
     def get_all_backends(self):
         return self._backends_conf.sections()
 
-    def get_backend_config(self, backend):
-        if backend not in self._backends_conf:
-            self._backends_conf.add_section(backend)
+    def get_backend_config(self, backend_id):
+        if backend_id not in self._backends_conf:
+            self._backends_conf.add_section(backend_id)
 
         return SectionConfig(
-            f'Backend {backend}',
-            self._backends_conf[backend],
+            f'Backend {backend_id}',
+            self._backends_conf[backend_id],
             DEFAULTS['backend'],
             self.save_backends_config)