Preliminary documentation of provider interfaces

* Preliminary documentation of CallsCall and CallsMessageSource
* Documentation and cleaning up of CallsCallData
* Add data files and meson rules to create gtk-doc documentation
* Move some macros from util.h to calls-message-source.h

src/calls-message-source.c

@@ -24,6 +24,15 @@
 
 #include "calls-message-source.h"
 
+/**
+ * SECTION:calls-message-source
+ * @short_description: A source of messages for the user.
+ * @Title: CallsMessageSource
+ *
+ * All three of the main interfaces, #CallsProvider, #CallsOrigin and
+ * #CallsCall require #CallsMessageSource.  They use this interface's
+ * message signal to emit messages intended for display to the user.
+ */
 
 G_DEFINE_INTERFACE (CallsMessageSource, calls_message_source, G_TYPE_OBJECT);
 
@@ -37,29 +46,23 @@ static guint signals [SIGNAL_LAST_SIGNAL];
 static void
 calls_message_source_default_init (CallsMessageSourceInterface *iface)
 {
+  GType arg_types[2] = { G_TYPE_STRING, GTK_TYPE_MESSAGE_TYPE };
+
+  /**
+   * CallsMessageSource::message:
+   * @self: The #CallsMessageSource instance.
+   * @text: The message text.
+   * @type: The type of the message; error, warning, etc.
+   *
+   * This signal is emitted when an implementing-object needs to emit
+   * a message to the user.  The message should be suitable for
+   * presentation to the user as-is.
+   */
   signals[SIGNAL_MESSAGE] =
     g_signal_newv ("message",
 		   G_TYPE_FROM_INTERFACE (iface),
 		   G_SIGNAL_RUN_LAST,
 		   NULL, NULL, NULL, NULL,
 		   G_TYPE_NONE,
-		   2, calls_message_signal_arg_types());
-}
-
-
-GType *
-calls_message_signal_arg_types()
-{
-  static gsize initialization_value = 0;
-  static GType arg_types[2];
-
-  if (g_once_init_enter (&initialization_value))
-    {
-      arg_types[0] = G_TYPE_STRING;
-      arg_types[1] = GTK_TYPE_MESSAGE_TYPE;
-
-      g_once_init_leave (&initialization_value, 1);
-    }
-
-  return arg_types;
+		   2, arg_types);
 }