[gtkmm] C++11: Replace (void*)0 with nullptr.

[Murray Cumming <murrayc src gnome org>]

commit 4504487c736049b525a4c5b79ef22c1e1c933c67
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Jul 17 10:35:41 2015 +0200

    C++11: Replace (void*)0 with nullptr.

 gdk/src/pixbuf.ccg            |    2 +-
 gtk/src/cellrenderercombo.ccg |    2 +-
 gtk/src/treeview.ccg          |    2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)
---
diff --git a/gdk/src/pixbuf.ccg b/gdk/src/pixbuf.ccg
index b433299..58706d3 100644
--- a/gdk/src/pixbuf.ccg
+++ b/gdk/src/pixbuf.ccg
@@ -160,7 +160,7 @@ void Pixbuf::save_to_buffer(gchar*& buffer, gsize& buffer_size,
   gdk_pixbuf_save_to_buffer(
       const_cast<GdkPixbuf*>(gobj()), &buffer, &buffer_size,
       type.c_str(),
-      &gerror, (void*)0);
+      &gerror, nullptr);
 
   if(gerror)
     ::Glib::Error::throw_exception(gerror);
diff --git a/gtk/src/cellrenderercombo.ccg b/gtk/src/cellrenderercombo.ccg
index bfdaa16..0aaf89f 100644
--- a/gtk/src/cellrenderercombo.ccg
+++ b/gtk/src/cellrenderercombo.ccg
@@ -25,7 +25,7 @@ static GtkTreeModel* _get_model(GtkCellRendererCombo* renderer)
     return 0;
 
   GtkTreeModel* combo_model = nullptr;
-  g_object_get(G_OBJECT(renderer), "model", &combo_model, (void*)0);
+  g_object_get(G_OBJECT(renderer), "model", &combo_model, nullptr);
   return combo_model;
 }
 
diff --git a/gtk/src/treeview.ccg b/gtk/src/treeview.ccg
index 850e32a..86fd53a 100644
--- a/gtk/src/treeview.ccg
+++ b/gtk/src/treeview.ccg
@@ -199,7 +199,7 @@ bool TreeView::get_path_at_pos(int x, int y, TreeModel::Path& path) const
 int TreeView::insert_column(const Glib::ustring& title, CellRenderer& cell, int position)
 {
   return gtk_tree_view_insert_column_with_attributes(
-      gobj(), position, const_cast<char*>(title.c_str()), cell.gobj(), (void*)0); //Note that some compilers 
need the extra (void*) clue.
+      gobj(), position, const_cast<char*>(title.c_str()), cell.gobj(), nullptr); //Note that some compilers 
need the extra (void*) clue.
 }
 
 int TreeView::append_column(const Glib::ustring& title, CellRenderer& cell)