Helper functions for registering stock icons

[Jaap Haitsma <jaap haitsma org>]

Hi,

Many gnome applications now have some code to register gnome_icon_theme 
icons by name or just png files part of the project as stock icons.

Wouldn't it be handy to add a helper function to GTK to do this? This 
would remove quite some code duplication and would also make it much easier.

Jaap