Replace leftover gtk-doc syntax (#Type) with backquotes.

Seems that msvc does not like that.

Using GList is a bit lame, and makes the API more complicated to use
than necessary in the common case.

The only real use case for a GList is gtk_widget_add_mnemonic_label(),
and for that we can use the GValue-based API instead.

Fixes: #3343

We're missing a closing parenthesis.

We don't need as many functions to print out the property, relation, and
state of an accessible. Additionally, we should allow comparing the
accessible attributes with an expected value, and print out the real
accessible value if they do not match.

We want to test the accessibility API, as well as the implementation
inside each widget. For that, we should expose an API that lets us
verify that a GtkAccessible has a given role, as well as a given
property.

The API follows the pattern of other GTest API:

 - a macro to assert that a condition is respected
 - a function that prints out the error message in case of failure