widgets: Update tool item visibility code

"visible-vertical" and "visible-horizontal" attributes
of GtkActions were deprecated in GTK 3.10, and since
the GAction port, we don't set them anyway on actions.
This seems to have impacted the ability to toggle a toolbutton's
visibility when a toolitem's "show" property is changed.
This patch replaced the deprecated "visibile-vertical" and
"visible-vertical" properties with a single "visible" property,
which consistently updates when the eye icon is toggled in
GimpToolEditor.

app/widgets/gimptoolpalette.c

@@ -462,10 +462,8 @@ gimp_tool_palette_add_button (GimpToolPalette *palette,
   gtk_widget_show (GTK_WIDGET (tool_button));
 
   g_object_bind_property (tool_item,   "shown",
-                          tool_button, "visible-horizontal",
-                          G_BINDING_SYNC_CREATE);
-  g_object_bind_property (tool_item,   "shown",
-                          tool_button, "visible-vertical",
+                          tool_button, "visible",
+                          G_BINDING_DEFAULT |
                           G_BINDING_SYNC_CREATE);
 
   button = gtk_bin_get_child (GTK_BIN (tool_button));