This is just lots of renaming.
The interface remains private, so the public API does not change, apart
from removing the definition of the Interface object to avoid
subclassing.
The property is useless to set (it only allows 'initial', 'inherit' and
'unset' as values), but it is used to track changes to the icon theme.
And as such, it can ensure that widgets can track when they need to
reload icons.
https://bugzilla.gnome.org/show_bug.cgi?id=743341