Add boolean return value for gtk_text_tag_table_add()

The user doesn't need to check the return value, because if FALSE is
returned it is a programmer error. But it permits a nicer behavior for
gtk_text_buffer_create_tag() in case of failure.

https://bugzilla.gnome.org/show_bug.cgi?id=614717
gtk_text_buffer_create_tag(): returns NULL on failure

Returns NULL in case of a duplicated tag name in the tag table. It is
still a programmer error to duplicate a name, but if it happens the
behavior is a little nicer (and hopefully doesn't crash).

https://bugzilla.gnome.org/show_bug.cgi?id=614717