Make "reserved_names" a base widget property so it can be overridden

[atbinf]

As we build higher level widgets, it would be handy if you could add to the list of "reserved_names" . For example, I'd like to add an "export" link widget to my grid layout but have it left alone in terms of being set as a children element as well as being pushed through the validation machinery.

This would also solve the issue of having to update tw2.core when downstream widgets add reserved_names. (like how "newlink" was added to support tw2.forms). the new library could just extend the reserved_names in it's own code.