Refactor BaseEditor (of EnumEditor) from toolkit specific packages

Currently both Qt and Wx implementations of the `EnumEditor` (and similar friends) use a base class for mapping and the inverse mapping of values, and the base class contains logic that is in fact toolkit agnostic.
See:
https://github.com/enthought/traitsui/blob/649dbd38a744ec5c0602aec807584ec49fc6bc5f/traitsui/qt4/enum_editor.py#L44-L46

https://github.com/enthought/traitsui/blob/649dbd38a744ec5c0602aec807584ec49fc6bc5f/traitsui/wx/enum_editor.py#L54-L56

These two base classes have exactly the same purpose. They are repeated just so that they can inherit from the toolkit specific `Editor`. It should be possible to refactor this into a mixin class so that the logic is more reusable and changes to these logics don't have to happen twice (or more times if we had more toolkits).