Extra properties or accessor for GtkButton

[Jody Goldberg <jody gnome org>]

http://bugzilla.gnome.org/show_bug.cgi?id=70961

The current code hard codes stock icons as being centered within the
button.  This is unpleasant when aligning button.  There are 2
simple solutions

1) Add stock_[xy]_alignment properties to gtkbutton.
2) Add a gtkbutton_get_stock_alignment

The former is obviously more controlled than the later.  Would
either patch be accepted ? are there any preferences for which
approach to take ?