We were only setting the state transiently in ::draw, leading
to various drawing anomalies, such as labels not picking up
the appropriate color from BUTTON styles.
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
