Split disguised attribute into two

The disguised attribute started off as a way to indicate a typedef to a
structure pointer, e.g.

    typedef struct Foo* FooPtr;

Over the years, though, it started to include opaque structure types,
e.g.

    typedef struct _FooObject FooObject;
    typedef struct _FooObjectClass FooObjectClass;

This has led to issues in language bindings, code generators, and
documentation generators, which now have issues when dealing with both
pointer aliases and opaque types.

An initial attempt at fixing this mess in commit f606183a ended up
breaking Vala, and had to be reverted.

To avoid breaking existing users we can follow a similar approach to the
allow-none/nullable/optional solution:

1. introduce a new pair of attributes: "pointer" and "opaque"
2. deprecate the "disguised" attribute

The "pointer" attribute covers the case of pointer types.

The "opaque" attribute covers the case of opaque structured types.

See also: https://gitlab.gnome.org/GNOME/vala/-/issues/735

Fixes...