GtkWidget: Add gtk_widget_get_style_context().

There will be one GtkStyleContext per widget, at the moment its
lifetime is tied to the widget's, but it could be narrowed down
to GTK_WIDGET_REALIZED.

gtk/gtkwidget.c

@@ -58,6 +58,7 @@
 #include "gtkbuildable.h"
 #include "gtkbuilderprivate.h"
 #include "gtksizerequest.h"
+#include "gtkstylecontext.h"
 #include "gtkdebug.h"
 
 
@@ -670,6 +671,7 @@ static GQuark		quark_tooltip_markup = 0;
 static GQuark		quark_has_tooltip = 0;
 static GQuark		quark_tooltip_window = 0;
 static GQuark		quark_visual = 0;
+static GQuark           quark_style_context = 0;
 GParamSpecPool         *_gtk_widget_child_property_pool = NULL;
 GObjectNotifyContext   *_gtk_widget_child_property_notify_context = NULL;
 
@@ -783,6 +785,7 @@ gtk_widget_class_init (GtkWidgetClass *klass)
   quark_has_tooltip = g_quark_from_static_string ("gtk-has-tooltip");
   quark_tooltip_window = g_quark_from_static_string ("gtk-tooltip-window");
   quark_visual = g_quark_from_static_string ("gtk-widget-visual");
+  quark_style_context = g_quark_from_static_string ("gtk-style-context");
 
   style_property_spec_pool = g_param_spec_pool_new (FALSE);
   _gtk_widget_child_property_pool = g_param_spec_pool_new (TRUE);
@@ -13161,3 +13164,23 @@ _gtk_widget_set_height_request_needed (GtkWidget *widget,
 {
   widget->priv->height_request_needed = height_request_needed;
 }
+
+GtkStyleContext *
+gtk_widget_get_style_context (GtkWidget *widget)
+{
+  GtkStyleContext *context;
+
+  g_return_val_if_fail (GTK_IS_WIDGET (widget), NULL);
+
+  context = g_object_get_qdata (G_OBJECT (widget),
+                                quark_style_context);
+
+  if (G_UNLIKELY (!context))
+    {
+      context = g_object_new (GTK_TYPE_STYLE_CONTEXT, NULL);
+      g_object_set_qdata_full (widget, quark_style_context, context,
+                               (GDestroyNotify) g_object_unref);
+    }
+
+  return context;
+}

gtk/gtkwidget.h

@@ -36,6 +36,7 @@
 #include <gtk/gtkadjustment.h>
 #include <gtk/gtkstyle.h>
 #include <gtk/gtksettings.h>
+#include <gtk/gtkstylecontext.h>
 #include <atk/atk.h>
 
 G_BEGIN_DECLS
@@ -941,6 +942,9 @@ void         _gtk_widget_buildable_finish_accelerator (GtkWidget *widget,
 
 gboolean     gtk_widget_in_destruction (GtkWidget *widget);
 
+GtkStyleContext * gtk_widget_get_style_context (GtkWidget *widget);
+
+
 G_END_DECLS
 
 #endif /* __GTK_WIDGET_H__ */