New varobj language callback: value_is_changeable_p.

New varobj language callback: value_is_changeable_p.

This patch introduces a new language-specific callback for varobj
objects, allowing us to move the language-specific bits of the
varobj_value_is_changeable_p routine to language-specific functions.
This is more elegant than testing for the varobj's language...

gdb/ChangeLog:

        * varobj.c (default_value_is_changeable_p): New function,
        extracted from varobj_value_is_changeable_p.  Add declaration.
        (ada_value_is_changeable_p): New function, extracted from
        varobj_value_is_changeable_p.  Add declaration.
        (struct language_specific): New field "value_is_changeable_p".
        (languages): Add entries for new field.
        (varobj_create): Set language before calling install_new_value.
        (varobj_value_is_changeable_p): Reimplement to call the varobj's
        "value_is_changeable_p" language callback.