[gtkmm] treeview demo: Use std::stoi() instead of atod() and c_str().

[Murray Cumming <murrayc src gnome org>]

commit 9a3ebfbbbe56f68646a3f6168bc937fbc7f9ad25
Author: Murray Cumming <murrayc murrayc com>
Date:   Tue Sep 1 12:43:48 2015 +0200

    treeview demo: Use std::stoi() instead of atod() and c_str().

 demos/gtk-demo/example_treeview_editable_cells.cc |    5 +----
 1 files changed, 1 insertions(+), 4 deletions(-)
---
diff --git a/demos/gtk-demo/example_treeview_editable_cells.cc 
b/demos/gtk-demo/example_treeview_editable_cells.cc
index 08614a2..4bf72de 100644
--- a/demos/gtk-demo/example_treeview_editable_cells.cc
+++ b/demos/gtk-demo/example_treeview_editable_cells.cc
@@ -9,8 +9,6 @@
 #include <cstdlib>
 #include <gtkmm.h>
 
-using std::strtod;
-
 class CellItem_Product
 {
 public:
@@ -204,8 +202,7 @@ void Example_TreeView_EditableCells::on_column_number_edited(const Glib::ustring
     if(iter)
     {
       //Convert the text to a number, using the same logic used by GtkCellRendererText when it stores 
numbers.
-      char* pchEnd = nullptr;
-      int new_value = (int) strtod(new_text.c_str(), &pchEnd);
+      auto new_value = std::stoi(new_text);
 
       //Store the user's new text in the model:
       Gtk::TreeRow row = *iter;