generator: Add support for suppressing the Equals(T) method in structs

Similarly to what exists for GetHashCode, add a noequals attribute to
indicate we don't want the default Equals implementation.
gtk: Use native function for GtkTextIter.Equals(GtkTextIter)

As GtkTextIter has no public properties, the default generated Equals
implementation would be "return true;". We now suppress that and with a
fix-up now use the native gtk_text_iter_equal to implement the Equals
method.