gcr: Hide the widget class structs, until we finalize their super classes.

gcr: Hide the widget class structs, until we finalize their super classes.

Hide the following GcrCertificateWidget, GcrCertificateWidgetClass,
GcrKeyWidget, GcrKeyWidgetClass, GcrUnlockOptionsWidget,
GcrUnlockOptionsWidgetClass

Currently these are derived from GtkAlignment, because we haven't yet
figured out their implementation correctly. We want them to be derived
from GtkWidget, or GtkBin (if it makes sense to have a child widget).