In iconic mode, model buttons will be styled like regular icon
buttons, preferring to show only the icon if one is set, falling
back to showing the label.
https://bugzilla.gnome.org/show_bug.cgi?id=727477
This is not a great name to use in themes; instead, add the
menuitem class, so themes can use .button.menuitem for styles
that are specific to menuitem-like buttons.
Similar in spirit to GtkModelMenuItem, this private GtkButton subclass
can connect to a GtkMenuTrackerItem and present itself as either a
regular button, a check button, or a radio button. Activation and
state tracking is done through the GAction that is associated with
the menu tracker item.
https://bugzilla.gnome.org/show_bug.cgi?id=723014
