Add generator expressions to menu

[MikeHopcroft]

A menu alias should be able to include a generator expression that produces a variety of aliases. For example, the expression
[small|large] [red|green] marble
should generate the following aliases
marble
green marble
small green marble
large green marble
red marble
small red marble
large red marble
Generator expressions should be able to denote optional items, required items, and choices.

[MikeHopcroft]

Implemented in commit 7f4ec73d.