Use standard Pyface/TraitsUI `Menu` objects for `TableEditor` extra menus

[corranwebster]

In addition to the standard context menu that is exposed in the TableEditor and used when the user clicks on a cell, there are two hard-coded menus provided by the editor: an "empty menu" that is shown when the user clicks in the main editor region but outside the standard rows, and a "header menu" which is shown when the user clicks in the vertical header.

These should be refactored to use the standard Menu (and/or Schema) infrastructure intially. Following that, either they should fold in the existing factory menu, or they should provide hooks in the factory for additional menu types on the factory object.