Add macros for GSourceFunc return values

The boolean values to be returned by a GSourceFunc are always ambiguous,
and even in case of experienced developers then can lead to confusion.

The Perl bindings for GLib have two simple constants, mapping to TRUE
and FALSE, that make the return values less confusing: G_SOURCE_CONTINUE
and G_SOURCE_REMOVE respectively.

https://bugzilla.gnome.org/show_bug.cgi?id=631413

docs/reference/glib/glib-sections.txt

@@ -446,6 +446,10 @@ G_PRIORITY_HIGH_IDLE
 G_PRIORITY_DEFAULT_IDLE
 G_PRIORITY_LOW
 
+<SUBSECTION>
+G_SOURCE_CONTINUE
+G_SOURCE_REMOVE
+
 <SUBSECTION>
 GMainContext
 g_main_context_new

glib/gmain.h

@@ -258,6 +258,26 @@ struct _GSourceFuncs
  */
 #define G_PRIORITY_LOW              300
 
+/**
+ * G_SOURCE_REMOVE:
+ *
+ * Use this macro as the return value of a #GSourceFunc to remove
+ * the #GSource from the main loop.
+ *
+ * Since: 2.28
+ */
+#define G_SOURCE_REMOVE         FALSE
+
+/**
+ * G_SOURCE_CONTINUE:
+ *
+ * Use this macro as the return value of a #GSourceFunc to leave
+ * the #GSource in the main loop.
+ *
+ * Since: 2.28
+ */
+#define G_SOURCE_CONTINUE       TRUE
+
 /* GMainContext: */
 
 GMainContext *g_main_context_new       (void);