modelize: introduce `AAttrPropdef::has_value`

Signed-off-by: Jean Privat <jean@pryen.org>
semantize: move noinit attribute to `AAttrPropdef::build_property`

Signed-off-by: Jean Privat <jean@pryen.org>
modelize: handle `autoinit` on attributes

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add base_init_autoinit2.nit

Signed-off-by: Jean Privat <jean@pryen.org>