APIs for setting GtkAction properties?

[Federico Mena Quintero <federico ximian com>]

GtkAction has a bunch of properties, including interesting ones like
"sensitive".  However, there are no corresponding functions to set
them.  So, it is hard to figure out what you can do to a property.

Should we add functions like gtk_action_set_sensitive(), etc.?  I can
cook a quick patch to do this.

  Federico