Gtk::Stock and buttons on win32

[Cedric Gustin <cedric gustin gmail com>]

This is a followup to a recent thread on the mailing list where people
complained about StockID not showing up in Gtk::Button on win32.

After investigating the issue, it turns out this depends on the
selected theme. With gtk-wimp (the default, native-like theme on
win32), icons are not displayed in buttons, even when they are created
using the Gtk::Button(const StockID& stock_id) constructor (the
equivalent of gtk_button_new_from_stock).

As an example, if you use the gladewin32 GTK+ installer, start the
"Theme Selector" tool. No icon is displayed in the "Ok" and "Cancel"
buttons. Now, change the theme to, say, "Metal", restart the theme
selector and you'll see the standard stock icons in the buttons.

Cedric