Move GtkSizeRequest into GtkWidget

It doesn't make sense to keep them separate as GtkSizeRequest requires a
GtkWidget and GtkWidget implements GtkSizeRequest, so you can never have
one without the other.
It also makes the code a lot easier because no casts are required when
calling functions.

Also, the names would translate to gtk_widget_get_width() and people
agreed that this would be a too generic name, so a "preferred" was added
to the names.

So this patch moves the functions:
gtk_size_request_get_request_mode() => gtk_widget_get_request_mode()
gtk_size_request_get_width() => gtk_widget_get_preferred_width()
gtk_size_request_get_height() => gtk_widget_get_preferred_height()
gtk_size_request_get_size() => gtk_widget_get_preferred_size()
gtk_size_request_get_width_for_height() =>
  gtk_widget_get_preferred_width_for_height()
gtk_size_request_get_height_for_width() =>
  gtk_widget_get_preferred_height_for_width()
... and moves the corresponding vfuncs to the GtkWidgetClass.

The patch also renames the implementations of the vfuncs in widgets to
include the word "preferrred".

gtk/gtkrecentchooserdefault.c

@@ -946,8 +946,7 @@ set_default_size (GtkRecentChooserDefault *impl)
   height = (impl->icon_size + font_size) * NUM_LINES;
 
   /* Use at least the requisition size... */
-  gtk_size_request_get_size (GTK_SIZE_REQUEST (widget),
-                             &req, NULL);
+  gtk_widget_get_preferred_size (widget, &req, NULL);
   width = MAX (width, req.width);
   height = MAX (height, req.height);
 
@@ -1849,8 +1848,8 @@ popup_position_func (GtkMenu   *menu,
   gdk_window_get_origin (gtk_widget_get_window (widget),
                          x, y);
 
-  gtk_size_request_get_size (GTK_SIZE_REQUEST (menu),
-                             &req, NULL);
+  gtk_widget_get_preferred_size (GTK_WIDGET (menu),
+                                 &req, NULL);
 
   gtk_widget_get_allocation (widget, &allocation);
   *x += (allocation.width - req.width) / 2;