gobject: add GBindingGroup

Originally, GBindingGroup started with Builder as a way to simplify all
of the third-degree object bindings necessary around Model-Controller
objects such as TextBuffer/TextView.

Over time, it has grown to be useful in a number of scenarios outside
of Builder and has been copied into a number of projects such as GNOME
Text Editor, GtkSourceView, libdazzle, and more.

It makes sense at this point to unify on a single implementation and
include that upstream in GObject directly alongside GBinding.

docs/reference/gobject/gobject-docs.xml

@@ -84,6 +84,7 @@
       <xi:include href="xml/gclosure.xml" />
       <xi:include href="xml/value_arrays.xml" />
       <xi:include href="xml/gbinding.xml" />
+      <xi:include href="xml/gbindinggroup.xml" />
   </reference>
   <reference label="III">
     <title>Tools Reference</title>

docs/reference/gobject/gobject-sections.txt

@@ -1006,3 +1006,21 @@ G_IS_BINDING
 g_binding_flags_get_type
 g_binding_get_type
 </SECTION>
+
+<SECTION>
+<FILE>gbindinggroup</FILE>
+GBindingGroup
+g_binding_group_new
+g_binding_group_dup_source
+g_binding_group_set_source
+g_binding_group_bind
+g_binding_group_bind_full
+g_binding_group_bind_with_closures
+<SUBSECTION Standard>
+G_TYPE_BINDING_GROUP
+G_TYPE_BINDING_GROUP_CLASS
+G_BINDING_GROUP
+G_IS_BINDING_GROUP
+<SUBSECTION Private>
+g_binding_group_get_type
+</SECTION>