Add macros and symbols for private data on dynamic types

Add macros and symbols for private data on dynamic types

We need a TypeName_private_offset variable defined by the macros used to
register dynamic types. We also need to call the adjust_private_offset()
function inside class_init(). G_ADD_PRIVATE_DYNAMIC only sets the size
of the private data structure, and relies on the behaviour of the
g_type_class_adjuset_private_offset() function to register the private
data structure at class init time if passed a value greater than zero.

This allows using G_PRIVATE_OFFSET with dynamic types.