TraitsUI Menus and Toolbars force import of backend

Because `Menu`, `MenuBar` and `ToolBar` are just renamed versions of equivalent Pyface objects which happen to be concrete toolkit objects, if you use these in a TraitsUI you end up choosing the backend at import time, which can be suboptimal.

In particular, the design of the rest of TraitsUI defers toolkit selection to the point where `edit_traits`\`configure_traits` is called, and this is important for allowing the writing of dual use CLI/GUI code.

Possible solutions include having proxy classes that defer creation of the actual instances, or perhaps using tasks menu/action schema that are designed for exactly this use case.