Add gtk_widget_add_tick_callback(), remove GtkTimeline, etc.

Add a very simple GtkWidget function for an "tick" callback, which
is connected to the ::update signal of GdkFrameClock.

Remove:

 - GtkTimeline. The consensus is that it is too complex.
 - GdkPaintClockTarget. In the rare cases where tick callbacks
    aren't sufficient, it's possible to track the
    paint clock with ::realize/::unrealize/::hierarchy-changed.

GtkTimeline is kept using ::update directly to allow using a GtkTimeline
with a paint clock but no widget.

gtk/gtkwidget.h

@@ -86,6 +86,21 @@ typedef 	GdkRectangle	   GtkAllocation;
 typedef void    (*GtkCallback)     (GtkWidget        *widget,
 				    gpointer          data);
 
+/**
+ * GtkTickCallback:
+ * @widget: the widget
+ * @frame_clock: the frame clock for the widget (same as calling gtk_widget_get_frame_clock())
+ * @user_data: user data passed to gtk_widget_add_tick_callback().
+ *
+ * Callback type for adding a function to update animations. See gtk_widget_add_tick_callback().
+ *
+ * Returns: %TRUE if the tick callback should continue to be called,
+ *  %FALSE if the tick callback should be removed.
+ */
+typedef gboolean (*GtkTickCallback) (GtkWidget     *widget,
+                                     GdkFrameClock *frame_clock,
+                                     gpointer       user_data);
+
 /**
  * GtkRequisition:
  * @width: the widget's desired width
@@ -905,12 +920,17 @@ void                    gtk_widget_insert_action_group                  (GtkWidg
                                                                          const gchar  *name,
                                                                          GActionGroup *group);
 
-GDK_AVAILABLE_IN_3_2
-void gtk_widget_add_frame_clock_target (GtkWidget           *widget,
-                                        GdkFrameClockTarget *target);
-GDK_AVAILABLE_IN_3_2
-void gtk_widget_remove_frame_clock_target (GtkWidget           *widget,
-                                           GdkFrameClockTarget *target);
+
+
+GDK_AVAILABLE_IN_3_8
+guint gtk_widget_add_tick_callback (GtkWidget       *widget,
+                                    GtkTickCallback  callback,
+                                    gpointer         user_data,
+                                    GDestroyNotify   notify);
+
+GDK_AVAILABLE_IN_3_8
+void gtk_widget_remove_tick_callback (GtkWidget       *widget,
+                                      guint            id);
 
 G_END_DECLS