[geary] Call g_binding_unbind directly, since it isn't bound for Vala < 0.26

[Robert Schroll <rschroll src gnome org>]

commit bfaeee6229a7f321cbc44322e3e4d824a0dbc815
Author: Robert Schroll <rschroll gmail com>
Date:   Wed Mar 11 21:03:04 2015 -0400

    Call g_binding_unbind directly, since it isn't bound for Vala < 0.26
    
    https://bugzilla.gnome.org/show_bug.cgi?id=746057

 src/client/components/main-toolbar.vala |    2 +-
 src/client/composer/composer-box.vala   |    2 +-
 src/client/util/util-gtk.vala           |    6 ++++++
 3 files changed, 8 insertions(+), 2 deletions(-)
---
diff --git a/src/client/components/main-toolbar.vala b/src/client/components/main-toolbar.vala
index 99524fe..b6ebc9a 100644
--- a/src/client/components/main-toolbar.vala
+++ b/src/client/components/main-toolbar.vala
@@ -192,7 +192,7 @@ public class MainToolbar : Gtk.Box {
         remove(header);
         header.get_style_context().remove_class("titlebar");
         header.get_style_context().remove_class("geary-titlebar-right");
-        guest_header_binding.unbind();
+        GtkUtil.unbind(guest_header_binding);
         header.show_close_button = false;
         conversation_header.show();
     }
diff --git a/src/client/composer/composer-box.vala b/src/client/composer/composer-box.vala
index e737a78..d523ec6 100644
--- a/src/client/composer/composer-box.vala
+++ b/src/client/composer/composer-box.vala
@@ -78,7 +78,7 @@ public class ComposerBox : Gtk.Frame, ComposerContainer {
             GearyApplication.instance.controller.main_window.main_toolbar.remove_conversation_header(
                 composer.header);
         if (title_binding != null)
-            title_binding.unbind();
+            GtkUtil.unbind(title_binding);
         
         composer.state = ComposerWidget.ComposerState.DETACHED;
         composer.editor.focus_in_event.disconnect(on_focus_in);
diff --git a/src/client/util/util-gtk.vala b/src/client/util/util-gtk.vala
index 1eb5c1e..2b89a6e 100644
--- a/src/client/util/util-gtk.vala
+++ b/src/client/util/util-gtk.vala
@@ -120,4 +120,10 @@ void apply_style(Gtk.Widget widget, string style) {
     }
 }
 
+/**
+ * This is not bound in Vala < 0.26.
+ */
+[CCode(cname = "g_binding_unbind")]
+extern void unbind(Binding binding);
+
 }