Added GtkScrollablePolicy property to scrollable interface

This patch adds the GtkScrollablePolicy type property to GtkScrollable
and implements it in all subclasses. GtkScrolledWindow observes this
property to make a good guess about when to show/hide scrollbars for
height-for-width content.

Most scrollable children do not do height-for-width *yet* but
most certainly will (toolpalette, treeview, iconview, textview
widgets all TODO), for scrollable widgets that do have a minimum
and natural size, it's important for them to observe the state
of this property in order to properly drive the scroll adjustments
according to the desired GtkScrollablePolicy. This patch makes
GtkViewport do this.

Patch also adds tests/testscrolledwindow.c to display the effects
of this property.

gtk/gtkscrollable.c

@@ -63,6 +63,7 @@
 #include "config.h"
 
 #include "gtkscrollable.h"
+#include "gtktypeutils.h"
 #include "gtkprivate.h"
 #include "gtkintl.h"
 
@@ -106,6 +107,38 @@ gtk_scrollable_default_init (GtkScrollableInterface *iface)
                                GTK_TYPE_ADJUSTMENT,
                                GTK_PARAM_READWRITE | G_PARAM_CONSTRUCT);
   g_object_interface_install_property (iface, pspec);
+
+  /**
+   * GtkScrollable:hscroll-policy:
+   *
+   * Determines whether horizontal scrolling should commence once the scrollable 
+   * widget is allocated less than it's minimum width or less than it's natural width.
+   *
+   * Since: 3.0
+   */
+  pspec = g_param_spec_enum ("hscroll-policy",
+			     P_("Horizontal Scrollable Policy"),
+			     P_("How the size of the content should be determined"),
+			     GTK_TYPE_SCROLLABLE_POLICY,
+			     GTK_SCROLL_MINIMUM,
+			     GTK_PARAM_READWRITE);
+  g_object_interface_install_property (iface, pspec);
+
+  /**
+   * GtkScrollable:vscroll-policy:
+   *
+   * Determines whether vertical scrolling should commence once the scrollable 
+   * widget is allocated less than it's minimum height or less than it's natural height.
+   *
+   * Since: 3.0
+   */
+  pspec = g_param_spec_enum ("vscroll-policy",
+			     P_("Vertical Scrollable Policy"),
+			     P_("How the size of the content should be determined"),
+			     GTK_TYPE_SCROLLABLE_POLICY,
+			     GTK_SCROLL_MINIMUM,
+			     GTK_PARAM_READWRITE);
+  g_object_interface_install_property (iface, pspec);
 }
 
 /**
@@ -203,3 +236,88 @@ gtk_scrollable_set_vadjustment (GtkScrollable *scrollable,
 
   g_object_set (scrollable, "vadjustment", vadjustment, NULL);
 }
+
+
+/**
+ * gtk_scrollable_get_hscroll_policy:
+ * @scrollable: a #GtkScrollable
+ *
+ * Gets the horizontal #GtkScrollablePolicy.
+ *
+ * Return value: The horizontal #GtkScrollablePolicy.
+ *
+ * Since: 3.0
+ **/
+GtkScrollablePolicy
+gtk_scrollable_get_hscroll_policy (GtkScrollable *scrollable)
+{
+  GtkScrollablePolicy policy;
+
+  g_return_val_if_fail (GTK_IS_SCROLLABLE (scrollable), GTK_SCROLL_MINIMUM);
+
+  g_object_get (scrollable, "hscroll-policy", &policy, NULL);
+
+  return policy;
+}
+
+/**
+ * gtk_scrollable_set_hscroll_policy:
+ * @scrollable: a #GtkScrollable
+ * @policy: the horizontal #GtkScrollablePolicy
+ *
+ * Sets the #GtkScrollablePolicy to determine whether 
+ * horizontal scrolling should commence below minimum or
+ * below natural width.
+ *
+ * Since: 3.0
+ **/
+void
+gtk_scrollable_set_hscroll_policy (GtkScrollable       *scrollable,
+				   GtkScrollablePolicy  policy)
+{
+  g_return_if_fail (GTK_IS_SCROLLABLE (scrollable));
+
+  g_object_set (scrollable, "hscroll-policy", policy, NULL);
+}
+
+/**
+ * gtk_scrollable_get_vscroll_policy:
+ * @scrollable: a #GtkScrollable
+ *
+ * Gets the vertical #GtkScrollablePolicy.
+ *
+ * Return value: The vertical #GtkScrollablePolicy.
+ *
+ * Since: 3.0
+ **/
+GtkScrollablePolicy
+gtk_scrollable_get_vscroll_policy (GtkScrollable *scrollable)
+{
+  GtkScrollablePolicy policy;
+
+  g_return_val_if_fail (GTK_IS_SCROLLABLE (scrollable), GTK_SCROLL_MINIMUM);
+
+  g_object_get (scrollable, "vscroll-policy", &policy, NULL);
+
+  return policy;
+}
+
+/**
+ * gtk_scrollable_set_vscroll_policy:
+ * @scrollable: a #GtkScrollable
+ * @policy: the vertical #GtkScrollablePolicy
+ *
+ * Sets the #GtkScrollablePolicy to determine whether 
+ * vertical scrolling should commence below minimum or
+ * below natural height.
+ *
+ * Since: 3.0
+ **/
+void
+gtk_scrollable_set_vscroll_policy (GtkScrollable       *scrollable,
+				   GtkScrollablePolicy  policy)
+{
+  g_return_if_fail (GTK_IS_SCROLLABLE (scrollable));
+
+  g_object_set (scrollable, "vscroll-policy", policy, NULL);
+}