* glib/glibmm/error.[cc|h]: Add operator bool(). Add Glib::Value<Glib::Error>
which is needed in _WRAP_PROPERTY() in Gtk::MediaStream.
* glib/glibmm/objectbase.[cc|h]: Add get_base_type(), which is necessary for
one of the existing Value<> specializations to apply. ObjectBase is used
in _WRAP_PROPERTY() in Glib::Binding.
* glib/src/variant.[ccg|hg]: Add Glib::Value<VariantBase> which is
needed in _WRAP_PROPERTY() in Gio:Action.
* glib/src/varianttype.[ccg|hg]: Add Glib::Value<VariantType> which is
needed in _WRAP_PROPERTY() in Gio:Action.

All data types that occur in _WRAP_PROPERTY() must have correct Glib::Value<>
specializations. In most cases missing Value<> specializations are noticed
only during execution of code that calls an affected property_*() method.