Move the Root interface to a private header

We don't expect out of tree implementations of GtkRoot, and having the
interface structure private to the GTK code allows us to add virtual
functions involving private types.

gtk/gtkroot.h

@@ -34,20 +34,6 @@ G_BEGIN_DECLS
 GDK_AVAILABLE_IN_ALL
 G_DECLARE_INTERFACE (GtkRoot, gtk_root, GTK, ROOT, GtkWidget)
 
-/**
- * GtkRootIface:
- *
- * The list of functions that must be implemented for the #GtkRoot interface.
- */
-struct _GtkRootInterface
-{
-  /*< private >*/
-  GTypeInterface g_iface;
-
-  /*< public >*/
-  GdkDisplay * (* get_display)  (GtkRoot *self);
-};
-
 GDK_AVAILABLE_IN_ALL
 GdkDisplay * gtk_root_get_display (GtkRoot *self);
 

gtk/gtkrootprivate.h

@@ -5,6 +5,20 @@
 
 G_BEGIN_DECLS
 
+/**
+ * GtkRootIface:
+ *
+ * The list of functions that must be implemented for the #GtkRoot interface.
+ */
+struct _GtkRootInterface
+{
+  /*< private >*/
+  GTypeInterface g_iface;
+
+  /*< public >*/
+  GdkDisplay * (* get_display)  (GtkRoot *self);
+};
+
 enum {
   GTK_ROOT_PROP_FOCUS_WIDGET,
   GTK_ROOT_NUM_PROPERTIES