Add type annotation for properties

Add type annotation for properties

Add type annotation syntax for GObject properties. This makes
it possible to override the type of a property. For instance,
this will allow function pointers with a G_TYPE_POINTER type set
to be used from a language binding which reads the typelib information
in addition g_object_class_find_property.

https://bugzilla.gnome.org/show_bug.cgi?id=618318